I read/write fluent French (not native, but close enough for Duolingo purposes). I just went through the course to see what I could find.
It's quite common for correct answers not to be accepted. In particular, often some technically correct literal translation is accepted but the almost certainly correct colloquial translation is not.
For example: "Tu manges ça"
Literally this means "you eat that" or "you are eating that". However, since it's hard to imagine someone saying "you are eating that" in real life (why would you ever say that?) I interpret this as a question ("are you eating that?") or as a command to a child ("Eat that!" or a bit more literally, "you shall eat that!")
However, the accepted answer was indeed "You are eating that.".
By the way, this isn't even taking into account the fact that "ça" should quite commonly be translated as "this", not "that" -- the distinction between "this" and "that" isn't the same in French as it is in English, and it's totally wrong to assume that "ça" should always be translated as "that" and "ceci" as "this". In fact, I think "are you eating this?" is orders of magnitude more likely to be the correct translation of "tu manges ça" than "you are eating that.".
I've only shared one, but there are a million examples like this on DuoLingo.
There are also some translations that are just objectively wrong: "He reads the menu" should be "il lit la carte", but on DuoLingo it's "il lit le menu". Nobody says "le menu" for "menu" in France. (They do say that in Québec, but nowhere was it indicated that I should be answering in nonstandard Québec dialect. "il lit la carte" was marked wrong here even though it's what you would say in France, the primary French-speaking country in the world.)
Similar to the "ça" == "this" fallacy, DuoLingo seems to believe that any phrase in which "le" or "la" appears has to involve "the" in the English translation. "You help kids" is apparently wrong for "Tu aides les enfants". The expected translation was "you're helping the children". Yes, that's one possible meaning, but "les enfants" isn't necessarily definite here: "tu aides les enfants" can certainly mean that "you help children" (or "kids", see the paragraph below), in general, whereas "you're helping the kids" implies some fixed, well-defined set of kids.
Let alone French: there are even cases in which DuoLingo's writers don't seem to understand English. For example, any sentence with "les enfants" has to be translated with the overly formal "children": "kids" isn't accepted, even though I, a native English speaker, would say "kids" something like 99.9% of the time.
That makes more sense to me. When I used Duolinguo it was to get caught up on some grammar and sentence structure, not vocabulary (which I can look up if I need to, and starts coming back quickly anyway as soon as I start speaking French). So I didn't really pay attention to word choices, and didn't internalize them.
(also, as my other comments make clear, the fact that my first French immersion was in Québec probably explains a lot, if you think Duolinguo is more Québecois)
It's quite common for correct answers not to be accepted. In particular, often some technically correct literal translation is accepted but the almost certainly correct colloquial translation is not.
For example: "Tu manges ça"
Literally this means "you eat that" or "you are eating that". However, since it's hard to imagine someone saying "you are eating that" in real life (why would you ever say that?) I interpret this as a question ("are you eating that?") or as a command to a child ("Eat that!" or a bit more literally, "you shall eat that!")
However, the accepted answer was indeed "You are eating that.".
By the way, this isn't even taking into account the fact that "ça" should quite commonly be translated as "this", not "that" -- the distinction between "this" and "that" isn't the same in French as it is in English, and it's totally wrong to assume that "ça" should always be translated as "that" and "ceci" as "this". In fact, I think "are you eating this?" is orders of magnitude more likely to be the correct translation of "tu manges ça" than "you are eating that.".
I've only shared one, but there are a million examples like this on DuoLingo.
There are also some translations that are just objectively wrong: "He reads the menu" should be "il lit la carte", but on DuoLingo it's "il lit le menu". Nobody says "le menu" for "menu" in France. (They do say that in Québec, but nowhere was it indicated that I should be answering in nonstandard Québec dialect. "il lit la carte" was marked wrong here even though it's what you would say in France, the primary French-speaking country in the world.)
Similar to the "ça" == "this" fallacy, DuoLingo seems to believe that any phrase in which "le" or "la" appears has to involve "the" in the English translation. "You help kids" is apparently wrong for "Tu aides les enfants". The expected translation was "you're helping the children". Yes, that's one possible meaning, but "les enfants" isn't necessarily definite here: "tu aides les enfants" can certainly mean that "you help children" (or "kids", see the paragraph below), in general, whereas "you're helping the kids" implies some fixed, well-defined set of kids.
Let alone French: there are even cases in which DuoLingo's writers don't seem to understand English. For example, any sentence with "les enfants" has to be translated with the overly formal "children": "kids" isn't accepted, even though I, a native English speaker, would say "kids" something like 99.9% of the time.