I answered: "What does Hungarian lawyer do" and the correction was "What do Hungarian lawyers do?" - I don't see why plural?
Because the correct English in singular should have an article (What does THE Hungarian lawyer do). As you omitted the article DL assumed that you want to say in plural.
For Hungarian it is common to use singular words for a profession in general. It is not applicable for all cases but I cannot think of a rule right now. But the point is, the plural is not wrong and your typo in English made DL to correct your reply.