i18n: first review of locale-de-DE.json

- refine some wording (may be subject to discussion), fix some typos
- initial version was based on nl-NL, which seems to be old and is missing a lot
  of new fields. Add them by comparison with en and fr-FR.
- Fixed l10n format strings (date patterns)
- I'm really unsure about adressing the user informally all the time. fr-FR
  doesn't do that either and I feel it to be more respectful.
- add flag and entry in language selector
- TODO: left API doc in english. It is developer doc anyway and those are
  seldomly translated.