I think starting 'this gentleman....sounds fine. As so often it depends on the context, and we don't habe that. So it should be allowed. After all it does show we have understood the French which surely is the point.
I also answered 'this gentleman is he Vietnamese?' Grammatically it appears to be ok in english but it was marked wrong
'this man/gentleman, is he vietnamese?' is not unnatural or uncommon at all in english, i hear it all the time. should be accepted
So when is the country capitalized, only when used as a noun? Also, are countries usually masculine, or is there a mix?
Yup exactly - capitalized when it's a noun, so when referring to the country itself (e.g. "La France est un pays") or a person/people (e.g. "Un Américain à Paris").
Regarding country gender - it's a messy mix of different genders and plurality. Even worse, there are a bunch of different rules for what prepositions to use when you want to say "to" or "in" a country, based not only on gender and plurality but also whether the starting sound is a vowel or consonant, and case by case rules for island countries. E.g. see https://frenchcrazy.com/2013/10/gender-and-prepositions-of-countries-in-french.html/
when can we use " gentleman " and " sir " usually to speak and write ?
I often hear them at the hotel, but rarely by native english speakers ?