500 server error and debug won't create log file

after trying to change wplang to danish suddenly front and back end show the server errror. turning on wp debug does not result in a log file as described in the codex.
what i’ve done before the error is simply create the languages folder, upload .po an .mo files and changed wp-config to the correct code da_DK.
incredibly enough changing it back to ” won’t resolve the problem, i have deleted the languages folder, all to no avail.
any ideas how to find out what causes the error?