Handle scenario when the user preferred language header is not passed

The UI when loading asks i18n plugin which language translation to use
depending on the user preferred language header 'accept-language'.
This commit is to handle scenario where header is not passed. The algorithm
then chooses the default language.

src/ui/index.js

@@ -145,6 +145,11 @@ function translate(key) {
 async function getTranslationLanguage(acceptLanguages) {
   let langStr = '';
   let foundLang = false;
+
+  if (acceptLanguages === null || acceptLanguages.length <= 0) {
+    return DEFAULT_LANGUAGE;
+  }
+
   const acceptLangsLen = acceptLanguages.length;
   const registeredLanguages = await i18nPlugin.getRegisteredTranslationLanguages();