I am not talking about questions in C# and answers in Java. What should we do with posts written in languages that are not English, like this one (now deleted)?
In the past, I have flagged them for a moderator using the
Other: reason. The posts should be closed/temporarily deleted until they are edited.
Since it's just one among many, a comment should work: Tell them (preferably adding a link) that Stack Overflow uses English. It shouldn't be a serious issue though; much less than 1% of answers are in Spanish.
Personally, I feel we should gently remind people of it, since it is simply really hard for some folks.