"I want some tea."
Trying to skip through here and find it odd that they use こうちゃ instead of お茶
こうちゃ is black tea, お茶 is green tea. I wish Duolingo would specify between the two in their English sentences.
There are plenty of situations where one would not specify which kind of tea they want. In that case, I would think お茶 would be the preferred term, since by definition, it just means "tea" (i.e. it is the honorific お followed by the kanji for tea, 茶). こう茶 specifically means "black tea" if I'm not mistaken (i.e. a kanji for red, 紅, followed by the kanji for tea, 茶).
That's the one I'm familiar with, but 紅 is the one used for 紅茶 specifically. It means "deep red" according to Google Translate.
"Some" is omitted in the translation. The answers 紅茶は少しほしいです or 少し紅茶がほしいです should also be acceptable...
少し isn't exactly the same thing as "some". I think Duolingo is using "some" as more of a plural indeterminate article than a quantifier. If they want "少し", they'll probably say something like "I want a little bit of tea". Obviously, it doesn't map well to Japanese; 少し probably should be accepted anyway.
Why split the word for tea? I'm not even sure most Japanese would answer this correctly.
How polite is this? Is this what one would say in a real Japanese restaurant?
~がほしい expresses what you want, but it's not the same way you'd request something in a restaurant. ください will suffice.
E.g. お水ください = Some water please.
こうちゃください = Some black tea please.
This sentence would be used to tell someone that you want tea(or something else). Not to order at a restaurant
First about the word "こうちゃ" actually meaning black tea, and "です" is not that necessary in this sentence as well.
It should probably accept answers without です, but it makes sense to include it in the official answer, since the entire course thus far has been in formal form. Dropping it would indicate the informal conjugation of 欲しい, which would be inconsistent with all the other です and ます translations.
I remember my Japanese teacher telling us not to use ほしい with food or drinks, because you don't want to have it just because, you want to do something with it. So, wouldn't this one be better as お茶を飲みたい?