Of course deleting .languagechanger in the template.css doesn't help. It just deletes the way it is supposed to be styled. What you could do is to use
.languagechanger {display:none;}
While still in the source code, the languagechanger shouldn't appear anymore.
Without knowing which template you use, it is basically impossible to tell you where in the template files you can find the language changer.
For the template "default" you can find {LANGUAGECHANGER} in the survey.pstpl. But this can be different for each template.