Cleanup translation files

Translations might become stale and obsolete over time, let's cleanup
slowly:
* Remove empty translations files (those were added by accident in the
  past or content gets removed over time)
* Remove files that have less than 20 per cent translated content.

Change-Id: I0c3f6de436d342d032fa38ce2df8a90dd878ea08
Merge "Cleanup translation files"