What I think is being addressed here is what I have heard discussed about the difference between being "bilingual" and "bicultural".  A non-native speaker who has skill as an educator can teach a language and even the literature of a different nation or culture.  The native speaker who has skill as an educator can teach the language in the context of the culture in which the language is used.

Does the value of understanding another culture trump the value of having Bulgarians employed in teaching Bulgarians.  I'm sure there are many differences of opinion.