“If I ask a student a language-related question on something we have just studied, how long should I give them to answer before asking another student the same question?”
If you ask someone a question, give them at least 6-10 seconds to formulate their reply – they need time to think. So don’t think the long pause before they reply is embarrassing. If you need an indication of whether you are waiting too long, look at the reaction of the other students – are they showing signs of frustration?
Rather than saying: Do / Did you understand the question? make it seem that you are responsible in some way for the student not understanding. This will help to avoid them losing face. So you can say: Let me rephrase that and then ask the question in a different way. You could also direct the question to the whole class, so that again the student doesn’t lose face or feel embarrassed.
Bear in mind that students may not admit they have not understood a question, and even if they say have understood this doesn’t necessarily mean that they really have understood.
You can also give the student the option to “ask a friend” before they reply – the student can ask for support if needed from the student sitting next to them.