First bug I found is the incorrect handling of the german language setting de_du_utf8. When an user has selected de_du_utf8 as his personal language setting in Moodle then he gets the english strings from en_utf8 in MRBS. This should be wrong!
The language pack de_du_utf8 is an official language pack that is based on de_utf8. There are also the correct settings for the parentlang inside the file moodledata/lang/de_du_utf8. I found no other block that shows the wrong german strings in Moodle.
My first bug fix was to duplicate the language pack so I get to german packages de_utf8 and de_du_utf8 .... but this should not be the best thing. If you change some strings then you have to change them in two file.
Second bug with the language handling is the fact that MRBS does not show the german help file site_faq.html that is currently placed inside the folder lang/de_utf8/help