Flag highly idiomatic speech and sayings
It would be great if highly idiomatic speech was flagged, so for example, when I am asked to translate "I see him once in a while" into German, I would be warned that this is idiomatic and not try to translate each word into German (i.e., "ab und zu" is not remotely close to a literal translation of "once in a while")
I don't think this is practical or desirable, personally. There is a continuous spectrum between 'highly idiomatic' and 'literal' -type phrases, and to mark some as the former and some as the latter would require a line to be drawn by Duo which would be arbitrary and meaningless, and not part of any broader understanding of the language.
It's just part of learning the language to know when to use an idiom and when not, and as such you've got to rely on your own feel and knowledge for it, not necessarily have Duo tell you this. In your example, you could try "occasionally" there in lieu of "once in a while", and if Duo is good enough it should know those two expressions amount to the same. If not, flag it as "should be accepted". There's usuallyways around having to write in idioms, I've found.
while of course there is a continuum between 'highly idiomatic' and 'literal,' it does not follow that such a distinction is entirely arbitrary or meaningless. to translate "I see him" into German has a one-to-one word correspondence (ich sehe ihn) which mirrors English usage. "ab und zu" has almost no lexical or syntactical correspondence to "once in a while."
Learning idioms is indeed part of a language, and I do not agree with one of the comments that they should be removed. I like that they are part of duolingo. but being asked to translate English into an idiom you've never been exposed to, simply because you have been exposed to the constituent vocabulary is not a productive use of learning time and can be frustrating for some learners to be penalized for not being able to do so.
I completely agree. I encounter two or three idiomatic statements a day, and they are often the difference between passing and failing a lesson. Even my German friends look at these things and say, "that's so unfair." Presenting them completely out of the blue is just wrong. As a former teacher, I cannot imagine putting things like this on exam having never exposed people to the material. If they wanted to include a unit on idioms, that would be useful.
Actually, I've just now found a situation where a highly idiomatic phrase WAS suggested by Duo, so they actually do cater for that! I forgot about it until now.
Faced with "Tel est pris qui croyait prendre.", the meaning of which I didn't know, I noticed that the word "pris" was highlit, as in Duo is telling me it's a new word. Mousing over said word revealed the idiomatic phrase "hoisted by one's own petard". Voila!
So I guess we just have to be on the qui-vive for those hints, which Duo apparently sometimes supplies.