How do we know that Johan is THAT smart?

Hi! I just finished Monster and loved it, especially the character of Johan haha. But when I did some research about him, everyone says that he's incredibly smart, just like Light Yagami or L, for example.

However, we don't have many proofs of his genius, except for his social intelligence, do we? For example, yes, he speaks French and English perfectly, but he learned them in 13 months, I think. And as far as I know, we don't know how long he took to learn Latin, but that doesn't seem THAT genius, does it? I mean 13 months to learn two languages is, imo, possible for someone with great abilities.

So of course, it's incredible; he is incredibly smart, but is he as much as I see sometimes? Thank you!