Does that mean that gamla and fria are nouns ('the ancient one', 'the free one') rather than adjectives? Could you say 'Han är fria' to mean 'He is the free [one]', as opposed to just 'Han är fri' 'He is free'? Or is this only the case if someone or something is being directly addressed?
I guess I would have felt more comfortable with an English translation of You the ancient, you the free. It just seems that in English we pretty much always put the appropriate form of to be between a personal pronoun and an adjective (You are ancient.... ), or else convert to something more like a noun (the ancient). I can't really think of any situations other than inner-city slang (e.g. You bad!) where one might just juxtapose you and an adjective.
Well, I suppose faithful is an adjective, but it is also a noun, and it is that aspect of the word which I think is being used in the English translation of Adeste Fidelis, and then the joyful and triumphant are simply modifying that noun. As to devalanteriel's comment, I was not complaining about the Swedish usage of gamla or fria being put in direct apposition to du, acting as a noun when put in the definite form, it was that doing effectively the same thing in English by turning ancient (adjective) into the ancient (noun) wasn't accepted, and it seems to me that throwing the in front of the adjective does that for English where we can't show our purpose by changing the ending on the adjective to make it definite. Of course, ancient, as faithful, has a specifically noun form without throwing a the in front of it, but specifically referencing people of ancient times (except for a few obsolete specific uses), and is usually used in the plural, as in the ancients.
All that said, I probably spoke too quickly, being momentarily annoyed that my translation had not been accepted. The Swedish Duolingo is so good at considering natural alternatives, compared to say the Russian DL, that usually when something I try is marked wrong it is simply because I really made what I consider to be an error. Perhaps that is because Swedish and English are so similar, but I think in large part is because the moderators are so thorough and meticulous.
Well, I suppose faithful is an adjective, but it is also a noun, and it is that aspect of the word which I think is being used
Well, if 'faithful' is a noun in that case, it isn't preceded by an article, so it still serves as a example of the same construction as the English translation of this sentence —'ancient' and 'free' can equally be nouns without needing articles; in fact, any adjective at all becomes a noun if you treat it as one by the same rationale, so distinguishing between adjectives and nouns in these instances becomes something of a technicality.
I agree it's not a terribly common usage, but it is still good English, and bear in mind that it is being used here to translate a fragment of poetry, not an everyday utterance.