How do you say bad boy in french?

Le mechant garcon.Je suis en France.

*le garçon méchant

Yeah, it could be that, but it depends on the context. If you're talking about a little boy, or a dog or something then yeah "le garçon méchant" or even "le mauvais garçon"

If you're on about "look at that the size of that bad boy," (sounds a bit dodgy but you get the picture) then it stays the same. I've heard it in pop songs and stuff

If you're on about "i prefer to go out with bad boys" then one of "le racaille, le rude boy, le bad boy, le caillera, le gangsta..." etc.