In addition to using Duolingo for learning Ukrainian, I went back and dabbled in using it for learning Japanese. My Japanese is somewhere in an intermediate area where I can have conversations and read books, but I don’t do either one particularly well. So, any effort to shore up those skills is help, and Duolingo is frankly a pretty fun app to use. It doesn’t really help with my efforts toward the JLPT, but there’s no reason why I can’t do both (time permitting).
However, at times using Duolingo for Japanese has been a little frustrating, such as this question:

which I got wrong for (in my opinion) nitpicky reasons:

The problem is, in my opinion, not with Duolingo. It’s a terrific service and app, and I would recommend it to anyone. The trouble is how Japanese is taught, and by extension non-European languages are taught.
The way that Duolingo teaches Japanese strongly resembles the same college courses I took way back in college in the late 1990’s, when we memorized similarly staid phrases, and textbook-style sentences that aren’t really used. It’s grammatically correct to say このコンビニにはフライドポテトがありますか, but it’s not how it’s naturally used.
Japanese has a tendency to be very contextual compared to English. This drove me nuts for a long time until my listening skills caught up just enough to know what the gist of the conversation was, and I didn’t hvae to explicitly know who did what and where.
For example, using the sentence above, if the context is known, it’s perfectly fine to say ありますか which would mean “[this convenience store] are there [french fries] [here]?”.
But suppose the person at the counter didn’t know which item you were asking about. Is what thing here? In such a case, use the particle が (ga) to specify who, what, which or where. Is what thing here? French fries, are they here (e.g. do you carry french fries)? Hence フライドポテトがありますか If you’re talking to an employee at the convenience store, this would be sufficient because you’re obviously standing in the convenience store, and obviously not talking about some other store.
Suppose the listener doesn’t know which store you’re talking about. Then, you’d have to clarify what store you’re talking about, hence use the particles に (the target particle) and は (the subject of your sentence) together コンビニにはフライドポテトがありますか。Depending on context this can mean either “do convenience stores carry french fries” in general or “does the (mutually understood) convenience store carry french fries?”
Maybe you’re standing outside with a buddy and you’re wondering if this Lawson convenience store has fries, vs. that 7-11 across the street. Then after all we’ve discussed so far, you’d have to specify THIS store, to the exclusion of others, is the one you’re inquiring about. Hence このコンビニにはフライドポテトがありますか
You can see why a textbook sentence like this can feel really wordy to Japanese speakers. It makes sense in English, and probably other European languages as well,1 but feels pretty unnatural in Japanese.
Similarly, a normal conversation that I literally just had with my wife as I was typing this is:
Mrs: 今日は寒いね。
(today, compared to other days, is cold, isn’t it?)Me: でも、暑くなると思う。
(but, [I] think it will get hot)
My wife did specify a topic (today’s weather), but if you notice I never said “I” anywhere in reply. Simply by context, using the word 思う (omou, “to think”), it’s obvious that i am stating my opinion.
It’s not limited to casual conversation either between spouses. A similar sentence in a more formal setting, such as with one’s boss might sound something like:
Boss: 今日は寒いね。
(today, compared to other days, is cold, isn’t it?)Underling: でも、暑くなると思いますね。
(but, [I] think it will get hot)
The boss, being of higher social rank, is free to use more casual speech to his/her underlings, but the underling would reply back using more polite speech (思います, not 思う), and yet still would not need to specify “I” such as 私は or whatever.
This is the sort of thing that I really, really wish I had learned in Japanese classes ages ago, but hard to learn the hard way. Again, this isn’t informal, casual Japanese necessarily, it’s common-place skills you have to learn to speak Japanese and unlearn habits that happen with European languages: not specifying things you don’t know need (i.e. context matters), and being sensitive to social rank and politeness.
P.S. I’ve dabbled in Korean ages ago during the “KPop wave” a few years back, and I believe what I said above also applies to Korean as well.
1 I don’t know enough Ukrainian yet to know how accurately the “textbook” Ukrainian matches real life conversation, but thanks to Duolingo, I know more about “Auntie Toma” and her family than I ever wanted to know. 😅