I made a comment early on in the first units of the German tree, but I feel compelled to mention it here, where it might get more attention. German, of course, is a language where all nouns are capitalized (as well as formal pronouns) but currently Duo does not enforce this in responses to the exercises. This was very confusing in the early levels of units where new nouns are introduced, especially since there the exercises seemed to require it.
My concern is that this grammatical rule really ought to be enforced. As far as I have been able to gather from reading a number of sites on German language, even colloquial usage seems to require proper capitalization. Lack of enforcement of this might lead to lazy habits later on?
I realize this situation probably arises because it calls for different handling of nouns for German (and perhaps other languages with this rule). My thinking, though, is that the response handler could be overridden just for the languages requiring it, avoiding endless switching in the code. This is a useful application for the overloading feature of OO languages, and I am assuming Duo's servers are supported by some OOP tool.
I imagine it was to make it easier for mobile users to "text" their answers into the exercises. If so, this is a terrible mistake, IMO. If I were German, I might feel just a bit put off by individuals pushing that agenda. German, for all its challenges, is still a very beautiful language and deserves to be preserved as its own people use it. Duolingo itself is a very useful learning tool when properly implemented and used.
I do occasionally use the mobile app, but I find it is incomplete by comparison to the desktop (web) version. I usually just use mobile chrome browser for duolingo just like I do at home.