diff --git a/core/modules/locale/locale.bulk.js b/core/modules/locale/locale.bulk.js
index ba9986b..5cde1b0 100644
--- a/core/modules/locale/locale.bulk.js
+++ b/core/modules/locale/locale.bulk.js
@@ -14,9 +14,11 @@
         var $langcode = $form.find('.langcode-input');
         $form.find('.file-import-input')
           .on('change', function () {
-            var matches = $(this).val().match(/\.([\-\w]+)\.po/);
-            if (matches && $langcode.find('option[value="' + matches[1] + '"]').length) {
-              $langcode.val(matches[1]);
+            // If there is a language which prefix is on the filename before the
+            // extension, pre-select it on the language input.
+            var matches = $(this).val().match(/(|\.)([\-\w]+)\.po/);
+            if (matches && $langcode.find('option[value="' + matches[2] + '"]').length) {
+              $langcode.val(matches[2]);
             }
           });
       }