hi
i installed a few weeks ago a new os (english only) ond a BB 7290 and kept the german language files from the old version by installing both os versions on my PC and copying all files with _de in the filename from the old to the new OS version and then installing it on the BB using desktop manager (it prompted english and german language version in the apploader dialog).
since there was not a huge version jump, the system talks perfectly german.. no odd behavior so far
maybe u can try something similar?
cheers
tate