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

...ranslated into the users language

Implements #6835.
You could turn this into:

```javascript
    context.translatedLangs = StringUtils.format(Strings.EXTENSION_TRANSLATED_LANGS,
        info.metadata.i18n.map(function (value) {
            return getLocalizedLabel(value);
        }).sort(function (lang1, lang2) {
            return lang1.localeCompare(lang2);
        }).join(", ")
    );
```

I'm guessing that JSLint will complain about my formatting there.