We've found a relatively odd problem; since there's a difference between the de and de_DE locale, we think it's a glibc issue. On a rawhide system, install man-pages-de and run LANG=de man man in an rxvt or similar terminal -> incomplete output LANG=de_DE man man works LANG=de man man in text mode works
I'm closing this. It has defnitely been a problem with the encoding of the man pages in the first place. The man pages are nowadays supposed to be encoded in a certain way and the man program can handle this. No need to look at what that old, outdated code did.