If all you want to do is speak and listen, there's not much wrong with focusing mostly on audio. But you'll find that when you get past a particular amount of words and grammar concepts learned, it becomes more difficult to learn new ones because there's no easy way to look things up or reference things in a purely audio-based way of working. Duolingo will certainly teach you new words, but you can probably learn a whole lot faster if you supplement it with other things as well. Obviously, that also depends on the language you are learning and what resources exist for it. Spanish has a whole lot more going for it than Swahili, for example.

Depends on the braille table, mostly. Most latin-based braille tables will have mappings for letters that language doesn't use; most accented letters are in the US English table for example, so in the vast majority of cases you should be ok if you set your braille table to your target language in those cases. Where non-latin alphabets are concerned, those will almost certainly not work with a latin-based table so in those cases, setting your braille table to the target language manually is required.

To my knowledge, language tags don't actually influence braille tables at all, so in all cases this would be a manual process. I just updated the guide with some tools to make switching between tables easier, which includes the tip on VoiceOver Activities :)