Move friendly language names to language packs

[fschmenger]

The lang definition in app-conf currently looks like the following:

"lang": {
    "supported": {
      "en": "English",
      "de": "Deutsch"
    },
    "fallback": "en"
  }

This should be changed to something like the following and the friendly language names should be externalized into the language packs:

"lang": {
    "supported": [
      "en",
      "de"
    ],
    "fallback": "en"
  }

This is a follow up task to #205 and was discussed in #233.

Quoted from #233:

Regarding the 'lang' configuration option: As an adhoc decision I incorporated the friendly language name in the app-conf, e.g. "en": "English" . These are currently only displayed in the new language picker control. Should we localize the friendly language names itself? The reasoning behind not translating those, is that a user might have trouble to indentify his language, when the active locale is e.g. japanese. On the other hand it's no satisfying to introduce new texts in app-conf, when the purpose of this change-set is to get rid of those. Proposal: I could obtain the friendly language names via language packs (and we could still decide on not translating them) !?