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

...ranslated into the users language

Implements #6835.
Well, we usually use two strings everytime there's singular/plural, so I guess in this case it's the same.
And many (common) extensions already got translated, like Brackets Git, Theseus, Brackets Code Folding, HTML Skeleton, Brackets Todo, ... (OK, not too many, but those are very common)