add new languages: xh and zu; bug 1074330

do all of our locales (the ones in `marketplace-constants` and in `l10n.js`) refer to the RFC 4646 ones?

because the [manifest](https://developer.mozilla.org/en-US/Apps/Build/Manifest#default_locale) expects RFC 4646 ones.

anyway, we need to add these:

```json
  ...
  "bn-BD",
  "ja",
  "mk",
  "nb-NO",
  "nl",
  "pt-BR",
  "ro",
  "ru",
  "sk", 
  "sq",
  "sr",
  "sr-Latn",
  "zh-CN"
  ...
```

feel free to do in another commit.