--- /dev/null
+/**
+ * @file
+ * Locale behavior.
+ */
+
+(function ($, Drupal) {
+ /**
+ * 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}
+ *
+ * @prop {Drupal~behaviorAttach} attach
+ * Attaches behavior for preselecting language code based on filename.
+ */
+ Drupal.behaviors.importLanguageCodeSelector = {
+ attach(context, settings) {
+ const $form = $('#locale-translate-import-form').once('autodetect-lang');
+ if ($form.length) {
+ const $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.
+ const matches = $(this).val().match(/([^.][\.]*)([\w-]+)\.po$/);
+ if (matches && $langcode.find(`option[value="${matches[2]}"]`).length) {
+ $langcode.val(matches[2]);
+ }
+ });
+ }
+ },
+ };
+}(jQuery, Drupal));