Ticket #42574: 42574.5.diff

src/wp-includes/script-loader.php

@@ -357 +357 @@
         $scripts->add( 'mediaelement-migrate', "/wp-includes/js/mediaelement/mediaelement-migrate$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' ),