Also thanks for the hint. During one of the many migrations we did back in the days, some collations got f***** and I kind of struggle to find theses translation issues. In the language files it's correct but on the site it shows the html escape sequences. It's kind of weird.
We're going to finish all the things we still want to and have to do on the site. After that I'm considering sharing the theme and for that we would have to write a little documention. What's good is, that the code is rather well documented already. That is something which could make the documentation process easier.