It's about time for my weekly question.

I declare an attack on opponent's character A. It's legal. He reinforces with character B. I play "lost in translation" on the reinforcer...."target character can not reinforce other characters this turn"......does the reinforcer stay exhausted and breakthrough still occur? Or do I have to play this on character B before the attack?

yeah before because it doesn't say that character X or whatever loses reinforcement. Its the card's biggest detraction.

Well, it depends on what's meant in the question---you say that "He reinforces with character B," but it's hard to tell what timing you mean. If he played a reinforcement effect, and you allowed that effect to resolve, then it's too late for Lost in Translation to stop it. If he played a reinforcement effect, and you played Lost in Translation before that effect could resolve, you'll stop him from reinforcing.

From the Legion FAQ:

I mean he says "I'll reinforce with character B" and I say "in response I'll lost in translation on character B".....no reinforce?

Like I mentioned, you can prevent reinforcement so long as LiT's effect resolves before the resolution of whaever reinforcement effect you want to stop. If you play LiT at your first chance after he plays the reinforcement effect, you win that race.