Question for teachers or anyone:?

How competent should a foreign language teacher to be to teach K-12? For example, let's say that the teacher was certified, competent, generally very advanced in their area, and generally are a good teacher, but not a fluent speaker of the language. They can explain concepts well, and the children learn, etc., but as they are not a native speaker, they make grammatical mistakes when speaking or writing. Would this bother you?