Aphasia

You and up to eight other willing creatures are able to speak in a language only they understand. The creatures must be able to speak a language already, and can still choose to speak other languages. To anyone else it sounds like nonsense, and even spells that decipher any language cannot make sense of it.

Alternately, you can use this spell to curse an unwilling creature to speak in nonsense. Choose a creature within your reach and make a melee spell attack against it. On a hit, the creature must make a Charisma saving throw. If the creature fails this save, it can only speak in random words and phrases. At your choosing you can still comprehend its speech, but no one else can. Any ability or spell with a verbal component requires the creature to make a concentration check against your spell DC. On a failure, the ability or spell fails.