Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/locale/locale.bulk.js

@@ -0,0 +1,35 @@
+/**
+ * @file
+ * Locale behavior.
+ */
+
+(function ($, Drupal) {
+
+  "use strict";
+
+  /**
+   * Select the language code of an imported file based on its filename.
+   *
+   * This only works if the file name ends with "LANGCODE.po".
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.importLanguageCodeSelector = {
+    attach: function (context, settings) {
+      var $form = $('#locale-translate-import-form').once('autodetect-lang');
+      if ($form.length) {
+        var $langcode = $form.find('.langcode-input');
+        $form.find('.file-import-input')
+          .on('change', function () {
+            // If the filename is fully the language code or the filename
+            // ends with a language code, pre-select that one.
+            var matches = $(this).val().match(/([^.][\.]*)([\w-]+)\.po$/);
+            if (matches && $langcode.find('option[value="' + matches[2] + '"]').length) {
+              $langcode.val(matches[2]);
+            }
+          });
+      }
+    }
+  };
+
+})(jQuery, Drupal);