In regards to that language headcanon that everyone’s getting excited about…
Jack Fenton is not an idiot. He loves to blabber on about a lot of things, and he’s a bit clueless at times, but there are small clues throughout the series that he IS an intelligent man. Why would he build a Ghost Gabber if ghosts communicated in English? Remember, the series addresses a lot of modern beliefs about ghosts. Why would they forget EVPs, one of the most popular tools for ghost hunters?
Ghosts often don’t speak in sound that can be understood. Thus, the Ghost Gabber most likely records EVP and relays it instantly.
It translates Danny’s speech. Presumably, he has the ability to speak ghost, and just doesn’t realise when he does so.