Index: src/wp-includes/script-loader.php
===================================================================
--- src/wp-includes/script-loader.php	(revision 42201)
+++ src/wp-includes/script-loader.php	(working copy)
@@ -356,8 +356,18 @@
 	$scripts->add( 'mediaelement-core', "/wp-includes/js/mediaelement/mediaelement-and-player$suffix.js", array(), '4.2.6-78496d1', 1 );
 	$scripts->add( 'mediaelement-migrate', "/wp-includes/js/mediaelement/mediaelement-migrate$suffix.js", array(), false, 1);
 
+	/*
+	 * translators: Translate this to a language tag supported by MediaElement.js.
+	 * Do not translate into your own language.
+	 */
+	$mejs_language = __( 'mejs_language' );
+	if ( 'mejs_language' === $output || preg_match( '/[^a-z-]/', $output ) ) {
+		$mejs_language = is_admin() ? get_user_locale() : get_locale();
+		$mejs_language = strtolower( strtok( $mejs_language ), '_' ); // Use only the first part for compatibility.
+	}
+
 	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' => $mejs_language,
 		'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' ),