French was spoken pretty much in every country of Western Europe, at least among the noble, the rich and the powerful; and the French living near the Spanish, Italian and German borders probably knew these languages as well. Given the existence of the Triwizard Tournament, it is however likely that the European cooperation was quite developed in the magical community, therefore the students would have at least one language in common. I don't know which, of French or English, is the most probable; so you might choose either. Both are possible, although it would be a good idea to have the students from the two foreign schools to have foreign language classes.
Originally Posted by hermy_loves_ron