[RDY] Fix localization

The existing build process to deal with localization files was broken.

This fixes found errors, and once build is running again, updates po files to sync them to current code (3a68a4861adcc950cdbde709d4841f8ea0c52b12).

I was also going to remove all unused messages (in #~ lines at the end of po files), cause a lot of code has been removed and that woud certainly produce a good number of deleted lines.
But then I learnt that gettext uses all known message/translation pairs (even unused ones) to give a tentative translation for new messages until proper translation is done.
For many languages, it can be a long time until a native-speaker gives a proper translation, and something is better than nothing, so I decided to keep existing unused messages, as they certainly add value to the project, and reducing lines in non-source-code files is much less valuable than the extra translation precision.