[w3c/manifest] Add optional `translations` member (#676)

Sometimes it's difficult to find an app or game on a specific (especially, unpopular) language.

Conversely some apps (games) are available only in one language (e.g. Chinese).

Since we want PWAs to be presented in app stores, what about adding optional `"translations"`/`"languages"`/`"supported_languages"` array of strings member?

Something like:

```json
"translations": ["en","fr","it"]
```

It should help to find (filter/exclude) apps by supported languages.

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