first draft: locale instructions

[Lotterleben]

PR for readability & easier discussion. I'm guessing this should be integrated into https://github.com/Lotterleben/wutzrock-deployment, if at all, because honestly I think the whole copying *.po s back and forth is bonkers and will send us into inconsistency hell.

I'd propose:

1. maintaining the wutzrock-locale-DONOTDELETE branch in our Engelsystem fork as described here
2. maintaining other changes such as config.php changes in similar branches
3. building releases based off the upstream engelsystem, with our customizations applied on top (all the instructions on how to maintain and update these branches can go into the release building .md)
4. rolling out these releases