English, of course
Spanish (Castilian and Latin American)
Swahili
Jamaican Patoi
French
Italian
Cockney Rhyming Slang, if that counts
My favorite is Swahili. I know no local native speakers, and so learning like SSL is ludicrously hard for me. It rolls off my tongue smoother than English, though, feels more natural, if that makes sense. I've been told by everyone but Kenyans that this is weird. They said it's sensible.