I think a lot of it has to do with whether or not you’ve learned a foreign language before, and how related the new language is to something you’ve learned before.
Here’s my experience:
I’m currently on a 153 day streak learning French. I took a year of French in high school that didn’t stick and was worthless.
Separately, I did pick up fluent Mexican Spanish, though, due to years of marriage to a Mexican national and years of living in various parts of Mexico. Both being Romance languages, there are a lot of similarities, and written French was about 40% intelligible to me based on Spanish. For example, when I was living in China and wanted to fix my Belgian-made water heater, I was able to find and understand the French language version of the owners manual, and fix my problem without waiting two weeks for service.
About 154 days ago I was motivated to solidify my French because I was going to visit Quebec, but not just Montreal and Quebec City where English is plentiful, but rural Quebec, where I was led to understand that English wouldn’t be welcome.
Duolingo is not the normal way I like to learn languages. I like rules and explanations, and Duolingo doesn’t do that. It uses repetition, but it’s been pretty effective. My ability to read and comprehend written French is such that I’m able to subscribe to some Quebec-related subs on Reddit, and keep my head above water.
Reading isn’t the same as speaking, though, and reading isn’t the same as listening, either, which is where I think that Duolingo falls short.
Because I don’t have someone outside of Duolingo to converse with, I’m still at the “translate what I want to say” phase, rather than “rolling off the tongue” phase in French, other than for certain, canned expressions. “Je voudrais manger avec toi ce soir” is easy enough, but I really have to think about more complex things before I can say them. And, of course, my pronunciation sucks.
Basically, I need someone to talk to to take full advantage of what I’ve learned, which, judging by my ability to read, really is considerable.
And, no, although the French isn’t Quebec French, it’s not been an issue. Outside of Duolingo, I’ve learned some of the Quebec-specific details of the language. Obviously I would prefer Duolingo to have Canadian French, but honestly, it’s not that much of a struggle. If you speak Mexican Spanish, getting used to Madrid Spanish is amusing but not a burden.
So, how did I do in Quebec? As expected, in Quebec City and Montreal, everyone answered me in English. In smaller towns, people tolerated my French, and were nice when I desoled myself and had to revert to English. My studying was not a waste, and made the experience richer. I’m continuing the course, because I want to get better.
I also play around Duolingo with Spanish. Some of the higher levels are really challenging for me. Maybe I’ll study them in the future, just to be better. I can kind of speak German once I’ve practiced a bit, so early levels have been a good refresher. And because I have a friend from Brazil, I’ve tried a few levels of Brazilian, just to I can say things like I want an apple.
Some of you may know that my wife is Chinese. This is where Duolingo sucks. I’m not at all interested in learning Hangzi. Maybe in the future, but not now. I just want to speak and hear. Written Chinese is an entirely different language that doesn’t currently interest me, but I’m not given the option to study Chinese using the Romanized alphabet (“Pinyin”). My brain is hard-wired to Roman and Greek/Cyrillic letters, and I need an actual alphabet to learn. Duolingo is useless to me in this respect.
Sorry for the wall of text!