"Một cô gái"
Not sure if it's really an error. In many Southeast Asian languages (Khmer and Thai, too), in some cases (often in songs or poems) words that end in -k, -t, -p are sometimes lengthened by adding a corresponding nasal sound, such as -kng, -tn, -pm. Maybe this also sometimes happen in normal speech...
Cô is somewhat shorter than you'd get if pronounced goh. Leave the h away. Also be aware that "một" is pronounced differently in the south. In the north you'll hear the final t, in the south it will get swallowed and sound more like a c. Google Translate is probably not the best help you can get if learning Vietnamese.
In English, you always can call an adult human female a woman (a young woman or an unmarried woman if you want to be more precise), and this will always be more polite than calling her a girl. As for how much more polite and when ‘girl’ is acceptable instead, that is (at least in the U.S.) something that people are divided on, and in fact it can be used as a sociocultural shibboleth, almost a political question (PC, political correctness). But however one feels about ‘girl’, ‘woman’ is never wrong if she is an adult.
In Vietnamese, must she be married to be một người phụ nữ, or must she still be một cô gái?
And the irony is that Vietnamese unmarried females don't want to be called women but they are much happy to receive flowers and gifts from their male family members, friends and colleagues on the International Women's Day (8th March) and the Vietnamese Women's Day (20th October).
Many expressions in Vietnamese consist of at least two words (there are few words, if any, longer than 7 letters, so there's a limit to just how much you can express with just one word).
Con can mean animal as well. So con chó means dog, for example. Con trai means boy, con gái means girl (but so does cô gái, slightly different meaning). Cháu alone also means a child, but usually someone else's. It also means nephew or niece. I'd call my own child con (alone), a child I meet in the street I'd call cháu. Actually, I'd call anyone cháu who's young enough to be the same age as my own children. They in turn would call me chú, uncle.
For extra fun consider the word con sông, which means a river. Basically, things that move can be called con + something else. A mouse or rat is chuột, but is called con chuột to make it certain that it's the animal that's meant.
This is my understanding anyway, and it's what makes Vietnamese so charming in my view.
In normal conversation you'd address someone as con without anything else (or cháu if not your own or closely related). The Vietnamese use personal pronouns much more than names. They just replace the English you.
Does it make sense to say "Một cô gái" grammatically? I'm wondering because I heard in Vietnamese you have to include "người" in sentences to make them sound right. Otherwise they sound weird. So would it be better to say "Một cô gái" or "người con gái" or "Một người cô gái" or "Một người con gái"? Sorry for all the questions. I'm still trying to get my head around Vietnamese.