Aphasia

A creature of your choice must make an Intelligence saving throw. On a failure they lose the ability to understand one language of your choice in all aspects, thus hindering their ability to communicate and understand their comrades. In addition, creatures under the effects of this spell are unable to use "Verbal" components to cast spells. At the end of their turn they may make another Saving Throw. Creatures succeeding the saving throw are aware a spell was cast on them. 

At higher levels:  you may target one additional creature for every spell slot used above the first.