l10n: try really hard to match a language

templates/lang_redirect.js

@@ -5,6 +5,7 @@ var languages =
 function lang_redirect(base, logical_url) {
     let lang = navigator.languages
         .map(lang => lang.replace('-', '_'))
+        .concat(navigator.languages.map(lang => lang.split('-')[0]))
         .filter(lang => languages.includes(lang))
         .shift();