Index: src/wp-includes/locale.php
===================================================================
--- src/wp-includes/locale.php	(revision 37441)
+++ src/wp-includes/locale.php	(working copy)
@@ -359,8 +359,27 @@
 	 * Constructor which calls helper methods to set up object variables
 	 *
 	 * @since 2.1.0
+	 * @param null $lang to change the current locale
 	 */
-	public function __construct() {
+	public function __construct( $lang = null ) {
+		if ( null !== $lang ) {
+			require_once( ABSPATH . 'wp-admin/includes/translation-install.php' );
+			global $locale;
+
+			if( ! in_array( $lang, get_available_languages() ) ) {
+				if ( array_key_exists( $lang, wp_get_available_translations() ) ) {
+					$language = wp_download_language_pack( $lang );
+					if ( $language ) {
+						load_default_textdomain( $language );
+						$locale = $language;
+					}
+				}
+			} else{
+				load_default_textdomain( $lang );
+				$locale = $lang;
+			}
+		}
+
 		$this->init();
 		$this->register_globals();
 	}
Index: tests/phpunit/tests/locale/test-locale.php
===================================================================
--- tests/phpunit/tests/locale/test-locale.php	(nonexistent)
+++ tests/phpunit/tests/locale/test-locale.php	(working copy)
@@ -0,0 +1,53 @@
+<?php
+/**
+ * wordpress-develop.
+ * User: Paul
+ * Date: 2016-05-16
+ *
+ */
+
+if ( !defined( 'WPINC' ) ) {
+	die;
+}
+
+class test_Locale extends WP_UnitTestCase {
+
+	static $fr_lanf_files = array(
+		'admin-fr_FR.mo',
+		'admin-fr_FR.po',
+		'admin-network-fr_FR.mo',
+		'admin-network-fr_FR.po',
+		'continents-cities-fr_FR.mo',
+		'continents-cities-fr_FR.po',
+		'fr_FR.mo',
+		'fr_FR.po',
+	);
+
+
+	/**
+	 * pass a new locale into WP_locale and reset current locale
+	 */
+	function test_WP_locale(){
+
+		global $GLOBALS;
+
+		$GLOBALS['wp_locale'] = new WP_Locale( 'fr_FR' );
+
+		$this->assertEquals( 'fr_FR', get_locale() );
+
+		$this->assertEquals( 'dimanche', $GLOBALS['wp_locale']->weekday[0] );
+
+		foreach ( self::$fr_lanf_files as $file ) {
+			$this->assertEquals( true, file_exists( WP_LANG_DIR . '/' . $file ) );
+		}
+
+		// remove any fr lang files
+		$this->remove_lang_files();
+	}
+
+	function remove_lang_files() {
+		foreach ( self::$fr_lanf_files as $file ) {
+			$this->unlink( WP_LANG_DIR . '/' . $file );
+		}
+	}
+}