We usually use the nouns for nationalities in constructions like this one, and they're gendered. So the most common way of saying it would be Hon är svenska, han är svensk.
Native speakers disagree about to what extent you can use an adjective as well, but at least for svensk, it is also possible, so that Hon är svensk is not wrong. But the more traditionally correct, and still more common way of saying it, would be Hon är svenska.
Typo handling for English is done automatically by Duo, not by the Swedish team, so this isn't really anything we can do a lot about. I don't know why they're so hard on coffee, I've noticed that other users have this problem too, so if you want to campaign for them to change it, I'm sure you can find support – bring it up in the general English forums to get their attention.
It's a bit complicated... but it's originally from the same root as ålder, meaning "age" in modern Swedish.
In Old Swedish, alder meant "age", both as in "the stone age" and as in "his age", "old age", etc. The singular dative of that was aldri, and this was combined with a suffix -g which was used to create negative forms.
Hence, the literal meaning is "not in any age".
(Note: these words had many spelling variations before the language was standardised, so I just used the simplest ones.)