Remember it is always a good idea to make backups before modifying any files.
That said, heres your fix.
login as root
cd /usr/local/directadmin/data/users/
ls */user.conf | xargs perl -pi -e "s/language=oldlanguage/language=newlanguage/g"
chances are the old language would be "en" and the new language would be "sp", so to be more exact:
ls */user.conf | xargs perl -pi -e "s/language=en/language=sp/g"