Index: src/wp-admin/includes/translation-install.php
===================================================================
--- src/wp-admin/includes/translation-install.php	(revision 0)
+++ src/wp-admin/includes/translation-install.php	(working copy)
@@ -0,0 +1,204 @@
+<?php
+/**
+ * WordPress Translation Install Administration API
+ *
+ * @package WordPress
+ * @subpackage Administration
+ */
+
+
+/**
+ * Retrieve translations from WordPress Translation API.
+ *
+ * @since 4.0.0
+ *
+ * @param string       $type Type of translations. Accepts 'plugins', 'themes', 'core'.
+ * @param array|object $args Translation API arguments. Optional.
+ * @return object|WP_Error On success an object of translations, WP_Error on failure.
+ */
+function translations_api( $type, $args = null ) {
+	include( ABSPATH . WPINC . '/version.php' ); // include an unmodified $wp_version
+
+	if ( ! in_array( $type, array( 'plugins', 'themes', 'core' ) ) ) {
+		return	new WP_Error( 'invalid_type', __( 'Invalid translation type.' ) );
+	}
+
+	/**
+	 * Allows a plugin to override the WordPress.org Translation Install API entirely.
+	 *
+	 * @since 4.0.0
+	 *
+	 * @param bool|array  $result The result object. Default false.
+	 * @param string      $type   The type of translations being requested.
+	 * @param object      $args   Translation API arguments.
+	 */
+	$res = apply_filters( 'translations_api', false, $type, $args );
+
+	if ( false === $res ) {
+		$url = $http_url = 'http://api.wordpress.org/translations/' . $type . '/1.0/';
+		if ( $ssl = wp_http_supports( array( 'ssl' ) ) ) {
+			$url = set_url_scheme( $url, 'https' );
+		}
+
+		$options = array(
+			'timeout' => 3,
+			'body' => array(
+				'wp_version' => $wp_version,
+				'locale'     => get_locale(),
+				'version'    => $args['version'], // Version of plugin, theme or core
+			),
+		);
+
+		if ( 'core' !== $type ) {
+			$options['body']['slug'] = $args['slug']; // Plugin or theme slug
+		}
+
+		$request = wp_remote_post( $url, $options );
+
+		if ( $ssl && is_wp_error( $request ) ) {
+			trigger_error( __( 'An unexpected error occurred. Something may be wrong with WordPress.org or this server&#8217;s configuration. If you continue to have problems, please try the <a href="https://wordpress.org/support/">support forums</a>.' ) . ' ' . __( '(WordPress could not establish a secure connection to WordPress.org. Please contact your server administrator.)' ), headers_sent() || WP_DEBUG ? E_USER_WARNING : E_USER_NOTICE );
+
+			$request = wp_remote_post( $http_url, $options );
+		}
+
+		if ( is_wp_error( $request ) ) {
+			$res = new WP_Error( 'translations_api_failed', __( 'An unexpected error occurred. Something may be wrong with WordPress.org or this server&#8217;s configuration. If you continue to have problems, please try the <a href="https://wordpress.org/support/">support forums</a>.' ), $request->get_error_message() );
+		} else {
+			$res = json_decode( wp_remote_retrieve_body( $request ), true );
+			if ( ! is_object( $res ) && ! is_array( $res ) ) {
+				$res = new WP_Error( 'translations_api_failed', __( 'An unexpected error occurred. Something may be wrong with WordPress.org or this server&#8217;s configuration. If you continue to have problems, please try the <a href="https://wordpress.org/support/">support forums</a>.' ), wp_remote_retrieve_body( $request ) );
+			}
+		}
+	}
+
+	/**
+	 * Filter the Translation Install API response results.
+	 *
+	 * @since 4.0.0
+	 *
+	 * @param object|WP_Error $res  Response object or WP_Error.
+	 * @param string          $type The type of translations being requested.
+	 * @param object          $args Translation API arguments.
+	 */
+	return apply_filters( 'translations_api_result', $res, $type, $args );
+}
+
+/**
+ * Get available translations from the WordPress.org API.
+ *
+ * @since 4.0.0
+ *
+ * @see translations_api()
+ *
+ * @return array Array of translations, each an array of data. If the API response results
+ *               in an error, an empty array will be returned.
+ */
+function wp_get_available_translations() {
+	if ( ! defined( 'WP_INSTALLING' ) && false !== ( $translations = get_site_transient( 'available_translations' ) ) ) {
+		return $translations;
+	}
+
+	include( ABSPATH . WPINC . '/version.php' ); // include an unmodified $wp_version
+
+	$api = translations_api( 'core', array( 'version' => $wp_version ) );
+
+	if ( is_wp_error( $api ) || empty( $api['translations'] ) ) {
+		return array();
+	}
+
+	$translations = array();
+	// Key the array with the language code for now.
+	foreach ( $api['translations'] as $translation ) {
+		$translations[ $translation['language'] ] = $translation;
+	}
+
+	if ( ! defined( 'WP_INSTALLING' ) ) {
+		set_site_transient( 'available_translations', $translations, 3 * HOUR_IN_SECONDS );
+	}
+
+	return $translations;
+}
+
+/**
+ * Output the select form for the language selection on the installation screen.
+ *
+ * @since 4.0.0
+ *
+ * @param array $languages Array of available languages (populated via the Translation API).
+ */
+function wp_install_language_form( $languages ) {
+	$installed_languages = get_available_languages();
+
+	echo "<label class='screen-reader-text' for='language'>Select a default language</label>\n";
+	echo "<select size='14' name='language' id='language'>\n";
+	echo '<option value="" lang="en" selected="selected" data-continue="Continue" data-installed="1">English (United States)</option>';
+	echo "\n";
+
+	if ( defined( 'WPLANG' ) && ( '' !== WPLANG ) && ( 'en_US' !== WPLANG ) ) {
+		if ( isset( $languages[ WPLANG ] ) ) {
+			$language = $languages[ WPLANG ];
+			echo '<option value="' . esc_attr( $language['language'] ) . '" lang="' . esc_attr( $language['iso'][1] ) . '">' . esc_html( $language['native_name'] ) . "</option>\n";
+			unset( $languages[ WPLANG ] );
+		}
+	}
+
+	foreach ( $languages as $language ) {
+		printf( '<option value="%s" lang="%s" data-continue="%s"%s>%s</option>' . "\n",
+			esc_attr( $language['language'] ),
+			esc_attr( $language['iso'][1] ),
+			esc_attr( $language['strings']['continue'] ),
+			in_array( $language['language'], $installed_languages ) ? ' data-installed="1"' : '',
+			esc_html( $language['native_name'] ) );
+	}
+	echo "</select>\n";
+	echo '<p class="step"><span class="spinner"></span><input id="language-continue" type="submit" class="button button-primary button-large" value="Continue" /></p>';
+}
+
+/**
+ * Download a language pack.
+ *
+ * @since 4.0.0
+ *
+ * @see wp_get_available_translations()
+ *
+ * @param string $download Language code to download.
+ * @return string|bool Returns the language code if successfully downloaded
+ *                     (or already installed), or false on failure.
+ */
+function wp_download_language_pack( $download ) {
+	// Check if the translation is already installed.
+	if ( in_array( $download, get_available_languages() ) ) {
+		return $download;
+	}
+
+	if ( defined( 'DISALLOW_FILE_MODS' ) && DISALLOW_FILE_MODS ) {
+		return false;
+	}
+
+	// Confirm the translation is one we can download.
+	$translations = wp_get_available_translations();
+	if ( ! $translations ) {
+		return false;
+	}
+	foreach ( $translations as $translation ) {
+		if ( $translation['language'] === $download ) {
+			$translation_to_load = true;
+			break;
+		}
+	}
+
+	if ( empty( $translation_to_load ) ) {
+		return false;
+	}
+	$translation = (object) $translation;
+
+	require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
+	$skin = new Automatic_Upgrader_Skin;
+	$upgrader = new Language_Pack_Upgrader( $skin );
+	$translation->type = 'core';
+	/**
+	 * @todo failures (such as non-direct FS)
+	 */
+	$result = $upgrader->upgrade( $translation, array( 'clear_update_cache' => false ) );
+	return $translation->language;
+}
Index: src/wp-admin/includes/upgrade.php
===================================================================
--- src/wp-admin/includes/upgrade.php	(revision 29609)
+++ src/wp-admin/includes/upgrade.php	(working copy)
@@ -437,6 +437,9 @@
 	if ( $wp_current_db_version < 26691 )
 		upgrade_380();
 
+	if ( $wp_current_db_version < 29600 )
+		upgrade_400();
+
 	maybe_disable_link_manager();
 
 	maybe_disable_automattic_widgets();
@@ -1304,7 +1307,29 @@
 		deactivate_plugins( array( 'mp6/mp6.php' ), true );
 	}
 }
+
 /**
+ * Execute changes made in WordPress 4.0.0.
+ *
+ * @since 4.0.0
+ */
+function upgrade_400() {
+	global $wp_current_db_version;
+	if ( $wp_current_db_version < 29600 ) {
+		if ( ! is_multisite() && false === get_option( 'WPLANG' ) ) {
+			if (
+				defined( 'WPLANG' ) && ( '' !== WPLANG ) && ( 'en_US' !== WPLANG ) &&
+				in_array( WPLANG, get_available_languages() ) )
+			{
+				update_option( 'WPLANG', WPLANG );
+			} else {
+				update_option( 'WPLANG', '' );
+			}
+		}
+	}
+}
+
+/**
  * Execute network level changes
  *
  * @since 3.0.0
@@ -1419,7 +1444,7 @@
  */
 function maybe_create_table($table_name, $create_ddl) {
 	global $wpdb;
-	
+
 	$query = $wpdb->prepare( "SHOW TABLES LIKE %s", $wpdb->esc_like( $table_name ) );
 
 	if ( $wpdb->get_var( $query ) == $table_name ) {
@@ -2192,145 +2217,3 @@
 	dbDelta( $ms_queries );
 }
 endif;
-
-/**
- * Output the input fields for the language selection form on the installation screen.
- *
- * @since 4.0.0
- *
- * @see wp_get_available_translations_from_api()
- * 
- * @param array $languages Array of available languages (populated via the Translations API).
- */
-function wp_install_language_form( $languages ) {
-	$installed_languages = get_available_languages();
-
-	echo "<label class='screen-reader-text' for='language'>Select a default language</label>\n";
-	echo "<select size='14' name='language' id='language'>\n";
-	echo '<option value="" lang="en" selected="selected" data-continue="Continue" data-installed="1">English (United States)</option>';
-	echo "\n";
-
-	if ( defined( 'WPLANG' ) && ( '' !== WPLANG ) && ( 'en_US' !== WPLANG ) ) {
-		if ( isset( $languages[ WPLANG ] ) ) {
-			$language = $languages[ WPLANG ];
-			echo '<option value="' . esc_attr( $language['language'] ) . '" lang="' . esc_attr( $language['iso'][1] ) . '">' . esc_html( $language['native_name'] ) . "</option>\n";
-		}
-	}
-
-	foreach ( $languages as $language ) {
-		printf( '<option value="%s" lang="%s" data-continue="%s"%s>%s</option>' . "\n",
-			esc_attr( $language['language'] ),
-			esc_attr( $language['iso'][1] ),
-			esc_attr( $language['strings']['continue'] ),
-			in_array( $language['language'], $installed_languages ) ? ' data-installed="1"' : '',
-			esc_html( $language['native_name'] ) );
-	}
-	echo "</select>\n";
-	echo '<p class="step"><span class="spinner"></span><input id="language-continue" type="submit" class="button button-primary button-large" value="Continue" /></p>';
-}
-
-/**
- * Get available translations from the WordPress.org API.
- *
- * @since 4.0.0
- *
- * @see wp_remote_post()
- *
- * @return array Array of translations, each an array of data.
- */
-function wp_get_available_translations_from_api() {
-	$url = 'http://api.wordpress.org/translations/core/1.0/';
-	if ( wp_http_supports( array( 'ssl' ) ) ) {
-		$url = set_url_scheme( $url, 'https' );
-	}
-
-	$options = array(
-		'timeout' => 3,
-		'body' => array( 'version' => $GLOBALS['wp_version'] ),
-	);
-
-	$response = wp_remote_post( $url, $options );
-	$body = wp_remote_retrieve_body( $response );
-	if ( $body && $body = json_decode( $body, true ) ) {
-		$translations = array();
-		// Key the array with the language code for now
-		foreach ( $body['translations'] as $translation ) {
-			$translations[ $translation['language'] ] = $translation;
-		}
-		return $translations;
-	}
-	return false;
-}
-
-/**
- * Download a language pack.
- *
- * @since 4.0.0
- *
- * @see wp_get_available_translations_from_api()
- *
- * @param string $download Language code to download.
- * @return string|bool Returns the language code if successfully downloaded
- *                     (or already installed), or false on failure.
- */
-function wp_install_download_language_pack( $download ) {
-	// Check if the translation is already installed.
-	if ( in_array( $download, get_available_languages() ) ) {
-		return $download;
-	}
-
-	// Confirm the translation is one we can download.
-	$translations = wp_get_available_translations_from_api();
-	if ( ! $translations ) {
-		return false;
-	}
-	foreach ( $translations as $translation ) {
-		if ( $translation['language'] === $download ) {
-			$translation_to_load = true;
-			break;
-		}
-	}
-
-	if ( empty( $translation_to_load ) ) {
-		return false;
-	}
-	$translation = (object) $translation;
-
-	require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
-	$skin = new Automatic_Upgrader_Skin;
-	$upgrader = new Language_Pack_Upgrader( $skin );
-	$translation->type = 'core';
-	/**
-	 * @todo failures (such as non-direct FS)
-	 */
-	$upgrader->upgrade( $translation, array( 'clear_update_cache' => false ) );
-	return $translation->language;
-}
-
-/**
- * Load a translation during the install process.
- *
- * @since 4.0.0
- *
- * @see load_textdomain()
- *
- * @param string $translation Translation to load.
- * @return string|bool Returns the language code if successfully loaded,
- *                     or false on failure.
- */
-function wp_install_load_language( $translation ) {
-	if ( ! empty( $translation ) ) {
-		if ( in_array( $translation, get_available_languages() ) ) {
-			$translation_to_load = $translation;
-		}
-	}
-
-	if ( empty( $translation_to_load ) ) {
-		return false;
-	}
-
-	unload_textdomain( 'default' ); // Start over.
-	load_textdomain( 'default', WP_LANG_DIR . "/{$translation_to_load}.mo" );
-	load_textdomain( 'default', WP_LANG_DIR . "/admin-{$translation_to_load}.mo" );
-	return $translation_to_load;
-}
Index: src/wp-admin/install.php
===================================================================
--- src/wp-admin/install.php	(revision 29609)
+++ src/wp-admin/install.php	(working copy)
@@ -38,6 +38,9 @@
 /** Load WordPress Administration Upgrade API */
 require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
 
+/** Load WordPress Translation Install API */
+require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+
 /** Load wpdb */
 require_once( ABSPATH . WPINC . '/wp-db.php' );
 
@@ -181,7 +184,7 @@
 switch($step) {
 	case 0: // Step 0
 
-		if ( empty( $_GET['language'] ) && ( $languages = wp_get_available_translations_from_api() ) ) {
+		if ( empty( $_GET['language'] ) && ( $languages = wp_get_available_translations() ) ) {
 			display_header( 'language-chooser' );
 			echo '<form id="setup" method="post" action="?step=1">';
 			wp_install_language_form( $languages );
@@ -193,9 +196,9 @@
 
 	case 1: // Step 1, direct link or from language chooser.
 		if ( ! empty( $_REQUEST['language'] ) ) {
-			$loaded_language = wp_install_download_language_pack( $_REQUEST['language'] );
+			$loaded_language = wp_download_language_pack( $_REQUEST['language'] );
 			if ( $loaded_language ) {
-				wp_install_load_language( $loaded_language );
+				wp_switch_default_textdomain( $loaded_language );
 			}
 		}
 
@@ -212,7 +215,7 @@
 		break;
 	case 2:
 		if ( !empty( $_REQUEST['language'] ) ) {
-			$loaded_language = wp_install_load_language( $_REQUEST['language'] );
+			$loaded_language = wp_switch_default_textdomain( $_REQUEST['language'] );
 		} else {
 			$loaded_language = 'en_US';
 		}
Index: src/wp-admin/network/settings.php
===================================================================
--- src/wp-admin/network/settings.php	(revision 29609)
+++ src/wp-admin/network/settings.php	(working copy)
@@ -102,7 +102,7 @@
 					<input name="admin_email" type="email" id="admin_email" class="regular-text" value="<?php echo esc_attr( get_site_option( 'admin_email' ) ) ?>" />
 					<p class="description">
 						<?php _e( 'This email address will receive notifications. Registration and support emails will also come from this address.' ); ?>
-					</p>	
+					</p>
 				</td>
 			</tr>
 		</table>
@@ -165,7 +165,7 @@
 <?php echo esc_textarea( $limited_email_domains == '' ? '' : implode( "\n", (array) $limited_email_domains ) ); ?></textarea>
 					<p class="description">
 						<?php _e( 'If you want to limit site registrations to certain domains. One domain per line.' ) ?>
-					</p>	
+					</p>
 				</td>
 			</tr>
 
@@ -231,7 +231,7 @@
 <?php echo esc_textarea( get_site_option( 'first_comment' ) ) ?></textarea>
 					<p class="description">
 						<?php _e( 'The first comment on a new site.' ) ?>
-					</p>	
+					</p>
 				</td>
 			</tr>
 			<tr>
@@ -273,25 +273,34 @@
 			</tr>
 		</table>
 
-<?php
+		<?php
 		$languages = get_available_languages();
 		if ( ! empty( $languages ) ) {
-			$lang = get_site_option( 'WPLANG' );
-?>
-		<h3><?php _e( 'Language Settings' ); ?></h3>
-		<table class="form-table">
+			?>
+			<h3><?php _e( 'Language Settings' ); ?></h3>
+			<table class="form-table">
 				<tr>
 					<th><label for="WPLANG"><?php _e( 'Default Language' ); ?></label></th>
 					<td>
-						<select name="WPLANG" id="WPLANG">
-							<?php mu_dropdown_languages( $languages, get_site_option( 'WPLANG' ) ); ?>
-						</select>
+						<?php
+						$lang = get_site_option( 'WPLANG' );
+						if ( ! in_array( $lang, $languages ) ) {
+							$lang = '';
+						}
+
+						wp_dropdown_languages( array(
+							'name'      => 'WPLANG',
+							'id'        => 'WPLANG',
+							'selected'  => $lang,
+							'languages' => $languages,
+						) );
+						?>
 					</td>
 				</tr>
-		</table>
-<?php
-		} // languages
-?>
+			</table>
+			<?php
+		}
+		?>
 
 		<h3><?php _e( 'Menu Settings' ); ?></h3>
 		<table id="menu" class="form-table">
@@ -324,7 +333,7 @@
 			</tr>
 		</table>
 
-		<?php 
+		<?php
 		/**
 		 * Fires at the end of the Network Settings form, before the submit button.
 		 *
Index: src/wp-admin/options-general.php
===================================================================
--- src/wp-admin/options-general.php	(revision 29609)
+++ src/wp-admin/options-general.php	(working copy)
@@ -302,23 +302,41 @@
 </select></td>
 </tr>
 <?php do_settings_fields('general', 'default'); ?>
+
 <?php
-	$languages = get_available_languages();
-	if ( $languages ) :
-?>
+$languages = get_available_languages();
+if ( ! empty( $languages ) ) {
+	?>
 	<tr>
-		<th width="33%" scope="row"><label for="WPLANG"><?php _e('Site Language') ?></label></th>
+		<th width="33%" scope="row"><label for="WPLANG"><?php _e( 'Site Language' ); ?></label></th>
 		<td>
-			<?php wp_dropdown_languages( array(
+			<?php
+			$locale = get_locale();
+			// Check if current locale is still installed. en_US will fail this test, but that's okay.
+			if ( ! in_array( $locale, $languages ) ) {
+				$locale = '';
+			}
+
+			wp_dropdown_languages( array(
 				'name'      => 'WPLANG',
 				'id'        => 'WPLANG',
-				'selected'  => get_option( 'WPLANG' ),
+				'selected'  => $locale,
 				'languages' => $languages,
-			) ); ?>
+			) );
+
+			// Add note about deprecated WPLANG constant.
+			if ( defined( 'WPLANG' ) && ( '' !== WPLANG ) && $locale !== WPLANG && is_super_admin() ) {
+				?>
+				<p class="description">
+					<?php _e( '<strong>Note:</strong> The <code>WPLANG</code> constant in your <code>wp-config.php</code> file is deprecated.' ); ?>
+				</p>
+				<?php
+			}
+			?>
 		</td>
 	</tr>
-<?php
-	endif;
+	<?php
+}
 ?>
 </table>
 
Index: src/wp-admin/options.php
===================================================================
--- src/wp-admin/options.php	(revision 29609)
+++ src/wp-admin/options.php	(working copy)
@@ -151,7 +151,7 @@
 		$options = $whitelist_options[ $option_page ];
 	}
 
-	// Handle custom date/time formats
+	// Handle custom date/time formats.
 	if ( 'general' == $option_page ) {
 		if ( !empty($_POST['date_format']) && isset($_POST['date_format_custom']) && '\c\u\s\t\o\m' == wp_unslash( $_POST['date_format'] ) )
 			$_POST['date_format'] = $_POST['date_format_custom'];
@@ -180,6 +180,14 @@
 			}
 			update_option( $option, $value );
 		}
+
+		// Switch translation in case WPLANG was changed.
+		$language = wp_unslash( trim( $_POST[ 'WPLANG' ] ) );
+		if ( $language ) {
+			wp_switch_default_textdomain( $language );
+		} else {
+			unload_textdomain( 'default' );
+		}
 	}
 
 	/**
Index: src/wp-admin/setup-config.php
===================================================================
--- src/wp-admin/setup-config.php	(revision 29609)
+++ src/wp-admin/setup-config.php	(working copy)
@@ -32,8 +32,12 @@
 
 require( ABSPATH . 'wp-settings.php' );
 
-require( ABSPATH . 'wp-admin/includes/upgrade.php' );
+/** Load WordPress Administration Upgrade API */
+require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
 
+/** Load WordPress Translation Install API */
+require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+
 nocache_headers();
 
 // Support wp-config-sample.php one level up, for the develop repo.
@@ -88,7 +92,7 @@
 switch($step) {
 	case -1:
 
-		if ( empty( $_GET['language'] ) && ( $languages = wp_get_available_translations_from_api() ) ) {
+		if ( empty( $_GET['language'] ) && ( $languages = wp_get_available_translations() ) ) {
 			setup_config_display_header( 'language-chooser' );
 			echo '<form id="setup" method="post" action="?step=0">';
 			wp_install_language_form( $languages );
@@ -100,9 +104,9 @@
 
 	case 0:
 		if ( ! empty( $_REQUEST['language'] ) ) {
-			$loaded_language = wp_install_download_language_pack( $_REQUEST['language'] );
+			$loaded_language = wp_download_language_pack( $_REQUEST['language'] );
 			if ( $loaded_language ) {
-				wp_install_load_language( $loaded_language );
+				wp_switch_default_textdomain( $loaded_language );
 			}
 		}
 
@@ -136,7 +140,7 @@
 	break;
 
 	case 1:
-		$loaded_language = wp_install_load_language( $_REQUEST['language'] );
+		$loaded_language = wp_switch_default_textdomain( $_REQUEST['language'] );
 		setup_config_display_header();
 	?>
 <form method="post" action="setup-config.php?step=2">
@@ -176,7 +180,7 @@
 	break;
 
 	case 2:
-	$loaded_language = wp_install_load_language( $_REQUEST['language'] );
+	$loaded_language = wp_switch_default_textdomain( $_REQUEST['language'] );
 	$dbname = trim( wp_unslash( $_POST[ 'dbname' ] ) );
 	$uname = trim( wp_unslash( $_POST[ 'uname' ] ) );
 	$pwd = trim( wp_unslash( $_POST[ 'pwd' ] ) );
Index: src/wp-includes/l10n.php
===================================================================
--- src/wp-includes/l10n.php	(revision 29609)
+++ src/wp-includes/l10n.php	(working copy)
@@ -24,7 +24,7 @@
  * @return string The locale of the blog or from the 'locale' hook.
  */
 function get_locale() {
-	global $locale;
+	global $locale, $wp_local_package;
 
 	if ( isset( $locale ) ) {
 		/**
@@ -38,26 +38,34 @@
 	}
 
 	// WPLANG is defined in wp-config.
-	if ( defined( 'WPLANG' ) )
+	if ( defined( 'WPLANG' ) ) {
 		$locale = WPLANG;
+	}
 
+	if ( isset( $wp_local_package ) ) {
+		$locale = $wp_local_package;
+	}
+
 	// If multisite, check options.
 	if ( is_multisite() ) {
 		// Don't check blog option when installing.
-		if ( defined( 'WP_INSTALLING' ) || ( false === $ms_locale = get_option( 'WPLANG' ) ) )
-			$ms_locale = get_site_option('WPLANG');
+		if ( defined( 'WP_INSTALLING' ) || ( false === $ms_locale = get_option( 'WPLANG' ) ) ) {
+			$ms_locale = get_site_option( 'WPLANG' );
+		}
 
-		if ( $ms_locale !== false )
+		if ( $ms_locale !== false ) {
 			$locale = $ms_locale;
-	} elseif ( ! defined( 'WP_INSTALLING' ) ) {
+		}
+	} else {
 		$db_locale = get_option( 'WPLANG' );
-		if ( $db_locale ) {
+		if ( $db_locale !== false ) {
 			$locale = $db_locale;
 		}
 	}
 
-	if ( empty( $locale ) )
+	if ( empty( $locale ) ) {
 		$locale = 'en_US';
+	}
 
 	/** This filter is documented in wp-includes/l10n.php */
 	return apply_filters( 'locale', $locale );
@@ -523,23 +531,29 @@
  * @see load_textdomain()
  *
  * @since 1.5.0
+ *
+ * @param string $locale Optional. Locale to load. Defaults to get_locale().
  */
-function load_default_textdomain() {
-	$locale = get_locale();
+function load_default_textdomain( $locale = null ) {
+	if ( null === $locale ) {
+		$locale = get_locale();
+	}
 
-	load_textdomain( 'default', WP_LANG_DIR . "/$locale.mo" );
+	$return = load_textdomain( 'default', WP_LANG_DIR . "/$locale.mo" );
 
 	if ( ( is_multisite() || ( defined( 'WP_INSTALLING_NETWORK' ) && WP_INSTALLING_NETWORK ) ) && ! file_exists(  WP_LANG_DIR . "/admin-$locale.mo" ) ) {
 		load_textdomain( 'default', WP_LANG_DIR . "/ms-$locale.mo" );
-		return;
+		return $return;
 	}
 
-	if ( is_admin() || ( defined( 'WP_REPAIRING' ) && WP_REPAIRING ) )
+	if ( is_admin() || ( defined( 'WP_INSTALLING' ) && WP_INSTALLING ) || ( defined( 'WP_REPAIRING' ) && WP_REPAIRING ) ) {
 		load_textdomain( 'default', WP_LANG_DIR . "/admin-$locale.mo" );
+	}
 
 	if ( is_network_admin() || ( defined( 'WP_INSTALLING_NETWORK' ) && WP_INSTALLING_NETWORK ) )
 		load_textdomain( 'default', WP_LANG_DIR . "/admin-network-$locale.mo" );
 
+	return $return;
 }
 
 /**
@@ -818,26 +832,99 @@
 }
 
 /**
- * Language selector. More to come.
+ * Language selector.
  *
  * @since 4.0.0
  *
  * @see get_available_languages()
+ * @see wp_get_available_translations()
  *
  * @param array $args Optional arguments. Default empty array.
  */
 function wp_dropdown_languages( $args = array() ) {
-	if ( isset( $args['languages'] ) ) {
-		$languages = $args['languages'];
-	} else {
-		$languages = get_available_languages();
+	require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+
+	$args = wp_parse_args( $args, array(
+		'id'        => '',
+		'name'      => '',
+		'languages' => array(),
+		'selected'  => ''
+	) );
+
+	if ( empty( $args['languages'] ) ) {
+		return false;
 	}
 
+	$translations = wp_get_available_translations();
+
+	/*
+	 * $args['languages'] should only contain the locales. Find the locale in
+	 * $translations to get the native name. Fall back to locale.
+	 */
+	$languages = array();
+	foreach ( $args['languages'] as $locale ) {
+		if ( isset( $translations[ $locale ] ) ) {
+			$translation = $translations[ $locale ];
+			$languages[] = $translation;
+		} else {
+			$languages[] = array(
+				'language'    => $locale,
+				'native_name' => $locale,
+				'lang'        => '',
+			);
+		}
+	}
+
 	printf( '<select name="%s" id="%s">', esc_attr( $args['name'] ), esc_attr( $args['id'] ) );
-	echo '<option value="">en_US</option>';
+
+	// List installed languages.
+	echo '<option value="">English (United States)</option>';
 	foreach ( $languages as $language ) {
-		$selected = selected( $language, $args['selected'], false );
-		echo '<option value="' . esc_attr( $language ) .'"' . $selected . '>' . $language . '</option>';
+		$selected = selected( $language['language'], $args['selected'], false );
+		printf(
+			'<option value="%s" lang="%s"%s>%s</option>',
+			esc_attr( $language['language'] ),
+			esc_attr( $language['iso'][1] ),
+			$selected,
+			esc_html( $language['native_name'] )
+		);
 	}
+
 	echo '</select>';
 }
+
+/**
+ * Switch translation for default translation.
+ *
+ * @since 4.0.0
+ *
+ * @see load_default_textdomain()
+ *
+ * @param string $translation Translation to load.
+ *  string|bool Returns the language code if successfully loaded,
+ *              or false on failure.
+ */
+function wp_switch_default_textdomain( $translation ) {
+	if ( ! empty( $translation ) ) {
+		if ( in_array( $translation, get_available_languages() ) ) {
+			$translation_to_load = $translation;
+		}
+	}
+
+	if ( empty( $translation_to_load ) ) {
+		return false;
+	}
+
+	if ( $translation_to_load === get_locale() ) {
+		return false;
+	}
+
+	unload_textdomain( 'default' ); // Start over.
+	$result = load_default_textdomain( $translation_to_load );
+
+	if ( ! $result ) {
+		return false;
+	}
+
+	return $translation_to_load;
+}
Index: src/wp-includes/version.php
===================================================================
--- src/wp-includes/version.php	(revision 29609)
+++ src/wp-includes/version.php	(working copy)
@@ -11,7 +11,7 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 29188;
+$wp_db_version = 29600;
 
 /**
  * Holds the TinyMCE version
Index: wp-config-sample.php
===================================================================
--- wp-config-sample.php	(revision 29609)
+++ wp-config-sample.php	(working copy)
@@ -62,16 +62,6 @@
 $table_prefix  = 'wp_';
 
 /**
- * WordPress Localized Language, defaults to English.
- *
- * Change this to localize WordPress. A corresponding MO file for the chosen
- * language must be installed to wp-content/languages. For example, install
- * de_DE.mo to wp-content/languages and set WPLANG to 'de_DE' to enable German
- * language support.
- */
-define('WPLANG', '');
-
-/**
  * For developers: WordPress debugging mode.
  *
  * Change this to true to enable the display of notices during development.