So, instead of going through the motions of changing my keyboard, I have been just using the English letters sometimes, instead of using the Ukrainian letters all the time. Does that still make it Ukrainian?
Unicode makes a distinction between Cyrillic letters and Latin letters, even when they look alike. For example, Latin 'i' is assigned code point U+0069, while Cyrillic 'і' is U+0456; Latin 'a' is U+0061, while Cyrillic 'а' is U+0430. Unless DuoLingo is programmed to translate Latin characters into their Cyrillic visual counterparts, typing Latin letters instead of Cyrillic ones won't work.