I teach 90% in English, and up too 100% depending on the level of my students. I think understanding is more important than immersion. Moreover, keeping students engaged and interested is a difficult task, and when they lose understanding in class, they stop paying attention. In my low-level classes, I'll allow them to ask me a question in their native language if they clearly want to engage me but can't find the English words. Then, I will teach them how to say what they wanted to ask me in English. Everything is a learning/teaching tool, and nothing should ever be "off the table" if it can help the students make a connection. Like anything, though, it has to be used correctly. One thing I've noticed is that students are much more comfortable attempting English when they know that I will allow them to fall back on their native language after an honest effort.

Could you suggest some reasons why you might think so? I'm curious about your own opinion. Also how often should this be allowed to happen?