Is it true that beer stimulates lactation if i’m nursing a baby?

No. At one point, it was thought that hops (an ingredient in beer) might help, but alcohol inhibits the milk ejection reflex (the "let down") and impairs milk release. And we have to be careful about drinking alcohol while nursing. You want to make sure that you nurse the baby first, then have a drink, if you choose, so that the alcohol can be cleared from your system before the next feeding.
No. I've heard this, but i could find no evidence that it's true. One beer isn't going to hurt, so i don't mind if you'd like to try, but really there's no reason to think that beer increases milk supply.