I can switch around just by going into language and text and putting Deutsch the top. Quit sys pref. Reopen it an it is German.
Have your tried turning it off and on again.
Just switch around if it got stuck somehow it should just switch back. Maybe set it to French and restart then set it to english and restart.
Restart the sys pref app not the whole notebook.
I use German Date Format, a semi German keyboard but English is on the top of that list, because I like that English words are shorter and take less space in Menus. I understand both but only the Language & Text has any effect on Menu languages. If you cannot fix it there you need to do something like reinstall OSX or try repairing permission and the usual stuff.