Media: Further improvements to the handling of language codes that get passed to MediaElement.

This change means that only the leading portion of a locale code gets passed to MediaElement, removing problems that arise from locales such as `de_DE_formal` and `pt_PT_ao90`.

Props erich_k4wp, blobfolio, flixos90, ocean90, joemcgill, SergeyBiryukov.

Fixes #42574

Merges [42251] to the 4.9 branch.

Built from https://develop.svn.wordpress.org/branches/4.9@42252


git-svn-id: http://core.svn.wordpress.org/branches/4.9@42081 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/script-loader.php

@@ -357,7 +357,7 @@ function wp_default_scripts( &$scripts ) {
 	$scripts->add( 'mediaelement-migrate', "/wp-includes/js/mediaelement/mediaelement-migrate$suffix.js", array(), false, 1);
 
 	did_action( 'init' ) && $scripts->add_inline_script( 'mediaelement-core', sprintf( 'var mejsL10n = %s;', wp_json_encode( array(
-		'language' => strtolower( str_replace( '_', '-', is_admin() ? get_user_locale() : get_locale() ) ),
+		'language' => strtolower( strtok( is_admin() ? get_user_locale() : get_locale(), '_-' ) ),
 		'strings'  => array(
 			'mejs.install-flash'       => __( 'You are using a browser that does not have Flash player enabled or installed. Please turn on your Flash player plugin or download the latest version from https://get.adobe.com/flashplayer/' ),
 			'mejs.fullscreen-off'      => __( 'Turn off Fullscreen' ),

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9.1-alpha-42248';
+$wp_version = '4.9.1-alpha-42252';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.