Depends on the game. Most arcade or action games tend to have a lot of English or at least enough that you can play the game ok. NBA Jam is in English except for the messages between quarters. Street Fighter is in English except for the text between matches and at the end. Usually its when you get into adventure or RPG territory where you'll have the whole game be in Japanese only. As for gameplay, usually there aren't huge differences but there's stuff like Contra that adds a health bar, Bare Knuckle 3 is easier and has a boss with bad gay stereotypes, etc.
Thanks sir.
Yeah, I'm familiar with import gaming on the MD, I collect for the system since almost two decades now
and my collection - even though it isn't neither huge or exceptionally impressive - is entirely made of Japanese games (I own a MD2 and a MCD2, both unmodded).
My question - sorry I couldn't explain myself better - was about Japanese releases of originally Western-developed games. Titles such as NBA Jam, Eternal Champions, Thunderhawk on the Mega-CD, Jaguar XJ, Aladdin... I watched some videos of the Japanese version of Thunderhawk for the Mega-CD, and it seems like it's completely dubbed in Japanese, making me wonder if the mission objectives would still be clear and understandable, for instance.
I asked because on the one hand I'm still procrastinating the purchase of a Game Genie for my Mega Drive, and a similar device for the Mega-CD too, and on the other one, I often stumble across NTSC-J copies of Western games such as Mortal Kombat CD, Soulstar, Wolfchild, and I feel tempted to grab those, instead of an adapter basically.