Language question
 

This is a question to any non-Brits:

If I said that something is "mental", what would that mean to you?

I'm looking for a word that says "mad, extreme, insane, incredible, outrageous", but not sure if that translates.

Try a thesaurus:

ape, barmy, bats in the belfry, batty, berserk, bonkers*, cracked, crazed, cuckoo, daft, delirious, demented, deranged, dingy*, dippy, erratic, flaky, flipped, flipped out, freaked out, fruity, idiotic, insane, kooky, lunatic, mad, mad as a March hare*, mad as a hatter, maniacal, mental*, moonstruck, nuts, nutty, nutty as fruitcake, of unsound mind, out of one's mind, out of one's tree, out to lunch, potty, psycho, round the bend, schizo, screw loose, screwball*, screwy*, silly, touched*, unbalanced, unglued, unhinged*, unzipped, wacky

Bonkers sounds good....
Also: wako, deranged, bugging,

will "crazy" just do?

What about, Unfuckingbelievable or Infuckingcredible?