aren't you being a bit selfish :) Most Turkish people have an English keyboard, why should they bother themselves to type special charachters they already know. This won't change. But even in the "Turkish for English speakers" we accept them, we show a typo message there but you can make those mistakes as many as you want (although only 1 regular type of typo is allowed)
So do you think I am being selfish typing Turkish in these comments: is that why you replied to me in English? I think Turks who have got to the end of this course should try their reverse course and type comments there in English.
Re keyboards I admit I had not considered Turks already living in the USA, but yes you are right as they may have bought laptops there it may be a fiddle for them to type Turkish characters. I think the best solution would be to have an option in the user settings which would default to accepting the non-Turkish characters, but which people like me could set to reject them. However as that would depend on the central Duolingo team in the meantime I accept your argument to leave it as it is. In practice at my stage this does not make much difference in the words in the example sentences as I know most of the vocab already. But I would still like to implore native speakers to spell properly in their comments: I find it useful to try to read the comments and sometimes there are words I find hard to look up in the dictionary due to misspelling. Thus I have set my spell checker temporarily to English to check this post, in case any advanced learners are reading it.