Add a second argument to FormFunction to specify you want an image format:
CloudDeploy[ FormFunction[{{"x","Sprache eingeben:"}->"Language"},
GeoRegionValuePlot[(Tooltip[#1,Column[{CommonName[#1],#1}]]->#2)&@@@(LanguageData[Interpreter["Language"][#x],"Speakers"])]&,
"PNG" ] ]
This is actually a bug, FormFunction is supposed to automatically convert Graphics and similar expressions automatically to images, but this will be equivalent to what FormFunction will do.
