Spanish.

Yogurt played a bigger role, but the spanish was more of a running gag. It was stated early that yougurt actually plays a role in a spy's diet, but Mike knowing Russian and other eastern Eropean languages but failing at Spanish when Spanish is very prominent in his hometown, made it more of a gag so to speak.

Nobody knows what I do, until I don't do it.

I think the yogurt was more noticeable. Someone eats a yogurt in nearly every episode, Michael gets a yogurt basket, he's given a yogurt shake and then a lifetime free frozen yogurt membership, his mentor Card is first seen with a yogurt in his hand. Not knowing Spanish wasn't mentioned/seen as much, IMO.