"I am ninth at the bar."
Translation:Sono la nona al bar.
From what I have read, the definite article is certainly preferred (perhaps even mandatory) before ordinal numbers.
In English it's correct grammar to keep the article (and many people do. It adds emphasis)
"I am the ninth at the bar" is also correct in English, so I guess Italians (or at least Duo) keep the article.
I think this works like Portuguese, we (brazilians) say "sou O nono". There isn't a specific rule about this, it's just how we say.
I think both are fine but putting the 'il' is the more natural way of saying it in Italian.
The definite article is not absolutely necessary here and can be omitted (in my humble opinion). Its use varies on context and mostly depends on the emphasis the speaker wants to put on the numeral.
You are correct, "Sono il nono" (m) and "Sono la nona" (f). All because of the verb "essere" and all linking verbs I think... Must be, surely...
The English sentence should then be "I am the ninth at the bar." if you want to translate correctly.