Index: src/wp-admin/css/forms.css
===================================================================
--- src/wp-admin/css/forms.css	(revision 29648)
+++ src/wp-admin/css/forms.css	(working copy)
@@ -761,6 +761,13 @@
 	margin: -3px 3px;
 }
 
+.settings-php .language-chooser .spinner,
+.options-general-php .language-chooser .spinner {
+	float: none;
+	margin: 0 3px;
+	vertical-align: middle;
+}
+
 /* =Media Queries
 -------------------------------------------------------------- */
 
Index: src/wp-admin/network/settings.php
===================================================================
--- src/wp-admin/network/settings.php	(revision 29648)
+++ src/wp-admin/network/settings.php	(working copy)
@@ -10,6 +10,9 @@
 /** Load WordPress Administration Bootstrap */
 require_once( dirname( __FILE__ ) . '/admin.php' );
 
+/** WordPress Translation Install API */
+require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+
 if ( ! is_multisite() )
 	wp_die( __( 'Multisite support is not enabled.' ) );
 
@@ -19,6 +22,29 @@
 $title = __( 'Network Settings' );
 $parent_file = 'settings.php';
 
+/**
+ * Display JavaScript on the page.
+ *
+ * @since 4.0.0
+*/
+function network_settings_add_js() {
+?>
+<script type="text/javascript">
+jQuery(document).ready( function($) {
+	var languageSelect = $( '#WPLANG' );
+	$( 'form' ).submit( function() {
+		// Don't show a spinner for English and installed languages,
+		// as there is nothing to download.
+		if ( ! languageSelect.find( 'option:selected' ).data( 'installed' ) ) {
+			$( this ).find( '.language-chooser .spinner' ).css( 'display', 'inline-block' );
+		}
+	});
+});
+</script>
+<?php
+}
+add_action( 'admin_head', 'network_settings_add_js' );
+
 get_current_screen()->add_help_tab( array(
 		'id'      => 'overview',
 		'title'   => __('Overview'),
@@ -58,6 +84,16 @@
 		'illegal_names', 'limited_email_domains', 'banned_email_domains', 'WPLANG', 'admin_email',
 	);
 
+	// Handle translation install
+	if ( ! empty( $_POST['WPLANG'] ) && ( ! defined( 'DISALLOW_FILE_MODS' ) || ! DISALLOW_FILE_MODS ) ) {
+		require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
+
+		$language = wp_download_language_pack( $_POST['WPLANG'] );
+		if ( $language ) {
+			$_POST['WPLANG'] = $language;
+		}
+	}
+
 	foreach ( $options as $option_name ) {
 		if ( ! isset($_POST[$option_name]) )
 			continue;
@@ -275,7 +311,8 @@
 
 		<?php
 		$languages = get_available_languages();
-		if ( ! empty( $languages ) ) {
+		$translations = wp_get_available_translations();
+		if ( ! empty( $languages ) || ! empty( $translations ) ) {
 			?>
 			<h3><?php _e( 'Language Settings' ); ?></h3>
 			<table class="form-table">
@@ -288,12 +325,16 @@
 							$lang = '';
 						}
 
+						echo '<div class="language-chooser">';
 						wp_dropdown_languages( array(
-							'name'      => 'WPLANG',
-							'id'        => 'WPLANG',
-							'selected'  => $lang,
-							'languages' => $languages,
+							'name'         => 'WPLANG',
+							'id'           => 'WPLANG',
+							'selected'     => $locale,
+							'languages'    => $languages,
+							'translations' => $translations,
 						) );
+						echo '<span class="spinner"></span>';
+						echo '</div>';
 						?>
 					</td>
 				</tr>
Index: src/wp-admin/options-general.php
===================================================================
--- src/wp-admin/options-general.php	(revision 29648)
+++ src/wp-admin/options-general.php	(working copy)
@@ -9,6 +9,9 @@
 /** WordPress Administration Bootstrap */
 require_once( dirname( __FILE__ ) . '/admin.php' );
 
+/** WordPress Translation Install API */
+require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+
 if ( ! current_user_can( 'manage_options' ) )
 	wp_die( __( 'You do not have sufficient permissions to manage options for this site.' ) );
 
@@ -50,6 +53,15 @@
 					date : format.val()
 				}, function(d) { format.siblings('.spinner').hide(); format.siblings('.example').text(d); } );
 		});
+
+		var languageSelect = $( '#WPLANG' );
+		$( 'form' ).submit( function() {
+			// Don't show a spinner for English and installed languages,
+			// as there is nothing to download.
+			if ( ! languageSelect.find( 'option:selected' ).data( 'installed' ) ) {
+				$( this ).find( '.language-chooser .spinner' ).css( 'display', 'inline-block' );
+			}
+		});
 	});
 //]]>
 </script>
@@ -305,7 +317,8 @@
 
 <?php
 $languages = get_available_languages();
-if ( ! empty( $languages ) ) {
+$translations = wp_get_available_translations();
+if ( ! empty( $languages ) || ! empty( $translations ) ) {
 	?>
 	<tr>
 		<th width="33%" scope="row"><label for="WPLANG"><?php _e( 'Site Language' ); ?></label></th>
@@ -316,12 +329,16 @@
 				$locale = '';
 			}
 
+			echo '<div class="language-chooser">';
 			wp_dropdown_languages( array(
-				'name'      => 'WPLANG',
-				'id'        => 'WPLANG',
-				'selected'  => $locale,
-				'languages' => $languages,
+				'name'         => 'WPLANG',
+				'id'           => 'WPLANG',
+				'selected'     => $locale,
+				'languages'    => $languages,
+				'translations' => $translations,
 			) );
+			echo '<span class="spinner"></span>';
+			echo '</div>';
 
 			// Add note about deprecated WPLANG constant.
 			if ( defined( 'WPLANG' ) && ( '' !== WPLANG ) && $locale !== WPLANG ) {
Index: src/wp-admin/options.php
===================================================================
--- src/wp-admin/options.php	(revision 29648)
+++ src/wp-admin/options.php	(working copy)
@@ -151,8 +151,8 @@
 		$options = $whitelist_options[ $option_page ];
 	}
 
-	// Handle custom date/time formats.
 	if ( 'general' == $option_page ) {
+		// Handle custom date/time formats.
 		if ( !empty($_POST['date_format']) && isset($_POST['date_format_custom']) && '\c\u\s\t\o\m' == wp_unslash( $_POST['date_format'] ) )
 			$_POST['date_format'] = $_POST['date_format_custom'];
 		if ( !empty($_POST['time_format']) && isset($_POST['time_format_custom']) && '\c\u\s\t\o\m' == wp_unslash( $_POST['time_format'] ) )
@@ -163,6 +163,21 @@
 			$_POST['gmt_offset'] = preg_replace('/UTC\+?/', '', $_POST['gmt_offset']);
 			$_POST['timezone_string'] = '';
 		}
+
+		// Handle translation install.
+		if (
+			! empty( $_POST['WPLANG'] ) &&
+			( ! is_multisite() || is_super_admin() ) &&
+			( ! defined( 'DISALLOW_FILE_MODS' ) || ! DISALLOW_FILE_MODS )
+		) {
+			require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
+			require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+
+			$language = wp_download_language_pack( $_POST['WPLANG'] );
+			if ( $language ) {
+				$_POST['WPLANG'] = $language;
+			}
+		}
 	}
 
 	if ( $options ) {
Index: src/wp-includes/l10n.php
===================================================================
--- src/wp-includes/l10n.php	(revision 29648)
+++ src/wp-includes/l10n.php	(working copy)
@@ -845,21 +845,21 @@
  * @param array $args Optional arguments. Default empty array.
  */
 function wp_dropdown_languages( $args = array() ) {
-	require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
 
 	$args = wp_parse_args( $args, array(
-		'id'        => '',
-		'name'      => '',
-		'languages' => array(),
-		'selected'  => ''
+		'id'           => '',
+		'name'         => '',
+		'languages'    => array(),
+		'translations' => array(),
+		'selected'     => ''
 	) );
 
-	if ( empty( $args['languages'] ) ) {
-		return false;
+	$translations = $args['translations'];
+	if ( empty( $args['translations'] ) ) {
+		require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+		$translations = wp_get_available_translations();
 	}
 
-	$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.
@@ -882,20 +882,43 @@
 		}
 	}
 
+	// Remove installed languages from available translations.
+	$translations = array_diff_key( $translations, $languages );
+
 	printf( '<select name="%s" id="%s">', esc_attr( $args['name'] ), esc_attr( $args['id'] ) );
 
 	// List installed languages.
-	echo '<option value="" lang="en">English (United States)</option>';
+	$structure[] = '<optgroup label="' . esc_attr_x( 'Installed', 'translations' ) . '">';
+	$structure[] = '<option value="" lang="en" data-installed="1">English (United States)</option>';
 	foreach ( $languages as $language ) {
 		$selected = selected( $language['language'], $args['selected'], false );
-		printf(
-			'<option value="%s" lang="%s"%s>%s</option>',
+		$structure[] = sprintf(
+			'<option value="%s" lang="%s"%s data-installed="1">%s</option>',
 			esc_attr( $language['language'] ),
 			esc_attr( $language['lang'] ),
 			$selected,
 			esc_html( $language['native_name'] )
 		);
 	}
+	echo '</optgroup>';
 
+	// List available translations.
+	if ( ! empty( $translations ) && ( ! is_multisite() || is_super_admin() ) && ( ! defined( 'DISALLOW_FILE_MODS' ) || ! DISALLOW_FILE_MODS ) ) {
+		$structure[] = '<optgroup label="' . esc_attr_x( 'Available', 'translations' ) . '">';
+		foreach ( $translations as $translation ) {
+			$selected = selected( $translation['language'], $args['selected'], false );
+			$structure[] = sprintf(
+				'<option value="%s" lang="%s"%s>%s</option>',
+				esc_attr( $translation['language'] ),
+				esc_attr( $translation['lang'] ),
+				$selected,
+				esc_html( $translation['native_name'] )
+			);
+		}
+		$structure[] = '</optgroup>';
+	}
+
+	echo join( "\n", $structure );
+
 	echo '</select>';
 }