Seems like a weird question and I don't know their personal lives (maybe they just bought an English 3DS), so why are they. in most cases, Japanese is faster so how much slower is Japanese than English in this game?

potentially capture card reasons or something

They run both languages, but I'm assuming they also run English for more international competition. The category forces to play in English, so even if Japanese were faster (which I do not think is the case in newer gens), it doesn't matter.