What's the rule for capitalisation? When I mouseover the names of continents it suggests them in all-lowercase. Is that just a peculiarity of Duolingo?
There are no hard-and-fast rules for capitalisation.
Proper nouns such as names are usually capitalised (so I think "Azio" is pretty much always in caps), but derivatives may or may not be capitalised (so "Asian" might be "Azia" or "azia"; "English (the language)" may be "la angla" or "la Angla", etc.).
In fact, while the rules may not be "hard-and-fast" there are rules. Countries and continents are capitalized. Languages are not. "Azia" is capitalized slightly more often than "angla" is - specially in older texts, but when you eliminate the cases where "Angla" is part of a title or the first word in a sentence, I've only been able to turn up a few isolated examples of this. Bottom line, don't capitalize "la angla."