Improve translations

[lukasjuhrich]

There are multiple things we could improve with the translations:

1. Don't use punctuation like exclamation marks, as they put needless emotion into the text.
2. Be consistent with the casing.
3. Generate and update the translations with --no-wrap as indicated in the makefile
4. Remove old messages from the messages.pot template
5. Use the built-in plural capabilities where adequate

See also #214 for the idea of extracting/compiling messages in the CI.