Write your answer in a way that you think would be most helpful for the person who asked the question.
That's how it works on all StackExchange sites. As for whether to answer in all English, all Latin, a mix, or to bring in other languages, let's have no rule—and no finger-wagging, scolding, or "discouraging". Let's leave that choice to the judgement of the person writing the answer. We have upvotes and downvotes to let the community render an opinion one answer at a time.
Levels of writing
We can probably learn something from another successful StackExchange site, English Language Learners. There, people learning English are required to ask all questions in English and all answers are in English—even though many of the questioners are still working at a very basic level of English, and their questions frequently contain grammatical errors.
A good answer is written at a level of English appropriate to what was demonstrated in the question. Some English constructions, such as those with phrasal verbs, can be tricky for ESL learners to understand. So, with total beginners, I usually avoid phrases like "get over" or "get into". On the other hand, with some users (like LePressentiment, who is also here), sometimes I intentionally write a little above their level, since I know they're ready for it and interested in it. Sometimes that leads to another fruitful question.
Pictures
By the way, pictures often help enormously. Here's a fine example by J.R., one of ELL's leading contributors, that very concisely addresses a subtlety of English grammar that often confuses ESL learners from non-Germanic languages: the way a preposition often follows (rather than precedes) a noun that it's indicating a relation to. This was made for a user who had been struggling with very basic English grammar:
In this case, the troublesome preposition is above.
