Index: wp-includes/l10n.php
===================================================================
--- wp-includes/l10n.php	(revision 39221)
+++ wp-includes/l10n.php	(working copy)
@@ -947,6 +947,7 @@
  * The default directory is WP_LANG_DIR.
  *
  * @since 3.0.0
+ * @since 4.7.0 The results are now filtrable.
  *
  * @param string $dir A directory to search for language files.
  *                    Default WP_LANG_DIR.
@@ -955,7 +956,7 @@
 function get_available_languages( $dir = null ) {
 	$languages = array();
 
-	$lang_files = glob( ( is_null( $dir) ? WP_LANG_DIR : $dir ) . '/*.mo' );
+	$lang_files = glob( ( is_null( $dir ) ? WP_LANG_DIR : $dir ) . '/*.mo' );
 	if ( $lang_files ) {
 		foreach ( $lang_files as $lang_file ) {
 			$lang_file = basename( $lang_file, '.mo' );
@@ -966,7 +967,15 @@
 		}
 	}
 
-	return $languages;
+	/**
+	 * Filters the list of available language codes
+	 *
+	 * @since 4.7.0
+	 *
+	 * @param array $languages An array of language codes or an empty array.
+	 * @param string $dir A directory to search for language files.
+	 */
+	return apply_filters( 'get_available_languages', $languages, $dir );
 }
 
 /**