Jo (pronounced a bit like your without the r sound) is affirmative in lots of European languages, including Finnish and Czech (it's spelled joo in Finnish iirc)

Interesting. In colloquial (American?) English, "yo" is used to get someone's attention, or to you've heard someone asking if you're present.

E.g., "Yo! Doofenschmirtz! You're new inator has been delivered."

Or: "Is Phineas here?" "Yo!"

True but that's a diphthong, "jo" is just a long(ish) o vowel sound. A lot of English dialects simply don't have an equivalent sound so it's hard to spell it. It's the IPA "o" or close to it. In my accent (south East England) "your" is pronounced almost identically to "jo".