One of the very first ones, in fact: Digimon World 1. In like... the opening dialog with Jijimon.
Presumably this means that all Digimon are "speaking Digimon language", which humans would hear as their native language. So I guess Terriermon is speaking Digimon, but the -audience- hears it as Japanese. Wallace would hear it as English.
(This also implies that Hawkmon and Piyomon are trolls who thought making Sora and Miyako communicate via Borscht and Caviar instead of just translating for them was hilarious.)
