as an english speaker, i would pronounce it "chat-oh", but i'm open to correction

I don't know what the "official" pronunciation is, but I would say "Chat-o" is probably right.

At least here in colloquial "rolo" spanish people use to call "chato" (which would sound the same as "chatto") someone with a pug, snub nose

Linguist here. It would likely be pronounced with a flap/tap, i.e., it would rhyme with shadow

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