Convert a lot of strings to proper format strings

This PR is basically some gettext cleanup.
Concatenated strings are extremely translator-hostile.
So I rewrote those sections with string concatenation and replaced them with proper format strings.

This is further work for issue #4658. There are still some string concatenations remaining to close the issue, but I am not in the right mood to finish this. However, this PR is ready to be merged alread.

Maybe I or someone else will at someday finally finish this issue.