Support package.json i18n key and show a hint in case the extension is t...

...ranslated into the users language

Implements #6835.
Small nit. This can be done:
```
var locales = [lang1.locale, lang2.locale],
    isUserLang = locales.indexOf(lang);
```
And `isUserLang` is actually the index in the index in the array. so it needs a better name like `userLangIndex` or shorter if possible.