That could be the case for other games but doesn't quite work for the Explorers of Darkness/Time/Sky games, as that one explicitly had you as a human partnered with a Grovyle in the past.I actually take it that the Pokemon themselves aren't speaking human language, but rather as a Pokemon you understand what Pokemon are saying. Whatever summoned the player character into the PMD world would deem it useful that you have all the benefits of being a Pokemon as well as the human traits that are desired to help save the world from... whatever. (giant meteor, giant evil ice thing, Darkrai's machinations, etc.)
Granted, due to memory loss it's never explicitly stated that you and Grovyle could directly communicate in the same language in the past. Future. Future-past. Alternate universe. Timey-wimey ball of stuff.
Although Grovyle's dialogue implies it. But if my Grovyle just started talking to me with the voice of Dan Green, I wouldn't question it.