It is to be assumed that lectures and seminars at the host universities are mainly taught in the local language, even though many universities also offer a range of classes in English.
The Swiss University of Fribourg offers many classes in German.
Generally, your skill in the local language should be at least at a B1 level if it is also the language of instruction. That way you will be able to follow classes at the university and find your way around in everyday life.
If English is the language of instruction, there is no need for a certificate other than your high school graduation certificate.