Yes, ata, this seems to be the problem pointed out by rthor. Delete the Language Installation and do a Language Install with the correct files. .po and .mo files. Hopefully it will work then.
Sometime it helps logging out and in again after the language install.
If you still have problems, then run a Systems Diagnostics, found on the Setup tab on FrontAccounting. Paste it here so we can see if there are other problems.
/Joe