I have a foreign word that begins with the letter "u", and don't know whether to use "a" or "an". The word is "unitario". It's a word used mainly in Argentina to describe a T.V. show comprising unrelated fictions which are broadcasted weekly and are usually an hour long. So I guess it comes from "unit". But in Spanish the word is pronounced with the vowel sound "u" as in "uzi", only shorter. So, what's the rule here? Should I say "an unitario" (as in "an uzi") or "a unitario" (as in "a unit")?
Since you're referencing a word used in a different language that isn't a loan word, you should "respect" that language's pronunciation. So since unitario as used in Argentina isn't pronounced beginning with
[j] (y sound as in you in English), you need to say an unitario.
Additionally, it is helpful if you let the reader know the word is foreign. This is usually done by italicizing the word.
It all depends on weather you start the word with a "y" sound like "you" or an "uhh" sound like "oops". If you feel your lips back for the "y" as you say it, then use "a". Given, however, that in both Spanish and Italian the word is said with the "oo" sound then if you want to pronounce it correctly you should be using "an".
In fact, the Spanish themselves use "un unitario". (With a "ooh" sound.)