Since the working language of the site is English, I always try to write my answers so that an English-speaking person (with the Russian language background enough to ask questions here) would understand them. Not that I always succeed, still :)
As for the equivalents in any non-English language, it's always OK to provide them, even if the user you're answering does not necessarily speak them, as long as you explain them in detail in English. This also concerns Russian equivalents.
For isntance, if someone would ask:
What does дать на лапу mean?
, an answer I would give would be something like
It means to bribe someone.
Literally, it means "to give (a bribe) upon a paw", the word взятка (bribe) being omitted here (a such called euphemistic ellipsis). It's similar to Sicilian fari vagnari a pizzu ("to wet someone's beak") made famous in English by Mario Puzo, author of "The Godfather".
In my opinion, it should not be necessary to have a deep knowledge of either Russian or Sicilian to understand this answer.
In other words, the answer should require no deeper knowledge or Russian (or any other language except English) to understand it than it was required to ask it.
