Mermish

Mermish is the native language of the Merpeople. Albus Dumbledore is a known non-Merperson speaker, and Percy Weasley claimed that Barty Crouch Sr. could also speak it. There is also a possibility that Luna Lovegood can speak Mermish. Her father, Xenophilius Lovegood tells her not to suppress that ability during Bill Weasley and Fleur Delacour's wedding.

Mermish is a curious language that is specifically adapted for use underwater. When underwater it sounds much like English, or perhaps even like any other language (Harry Potter is the only one who personally shown in the books to hear the merpeople), however, out of water the language sounds very screechy and harsh. It is unknown whether those none-merpersons who can speak mermish sound the same as merpeople underwater. That is to say, it is unknown whether they would sound like English if speaking underwater, it is possible that only merpeople possess the unique throat piece necessary to make sound underwater like merpeople do.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Deathly Hallows
 * Fantastic Beasts and Where to Find Them