Sorry about the link - https://lang.moodle.org of course. Dutch speaking, from Belgium
Good that those 2 possible causes are ruled out.
- not a local modification
- not a problem with the module
As for the local modification: you do use de and not de_du, de_wp, de_com, ...? Otherwise you have to check the other de_xx_local folders of course.
- I checked the language pack and verified that all quickmail strings are translated and German: OK
- I checked the derived language packs (comm, kids, du, wp, ch) to verify that there are no strings for quickmail there: OK
So we are now sure that the problem is not on the end of the language pack. It must be a local problem.
From the thread I got that the strings are there in your language pack. So I can see only 2 reasons why they are not used: they are overwritten by English or the system can't read/find the German files. You are sure folders in lang are all readable and executable for the webserver proces (e.g. 755) and the files are at least readable (e.g 644) ?
I'm very curious now because I honestly don't know where else to look. Going through the tread, there isn't much that isn't covered yet.