Camera/MP3 playback failure is probably down to you flashing a C902 xxxx_AS firmware when you have an older C902 that requires AO firmware. See my post Ao Or As Firmware - C902 - SE-NSE Forums - Sony Ericsson Forum and Community. I had to use the R3DA028 WEST-EUROPEAN firmware.
As for getting German on your phone I put Italian on my friend's phone by downloading a C902 ADRIATIC FS file (which has Italian). I then used DB3150 FS extractor from Db3150 Fs Extractor - SE-NSE Forums - Sony Ericsson Forum and Community to extract the language onto my PC. I then used A2uploader to put the language on the phone. Note, I think I had to ensure that the files were in lower case, i.e. it.lng and it.t9.