I used to be a student in a very similar situation. As I remember, the professor simply interrupted the question with the phrase: "In english please". Maybe the student does not have enough confidence to speak in english, otherwise, ignoring such a request seem to be highly disrespectful and you should not worry about being assertive with him.

The student has no right to be offended by requiring him to fulfill the course requirements - if the course is officially listed as being taught in English/its working language is English. In such a case I would just act dumb: the student asks a question in the native language, I respond in English "sorry, I do not understand". The purpose of teaching a course in a foreign language is for the students to gain practice and confidence. If the student thinks his English is not so good - that's the reason he should be practicing it. Of course, throwing in a few words for clarity would be ok imho.

Do you see communication of course material in English as an important part of the course (even if it isn't the primary purpose)? That is, is there some reason this course is taught in English, perhaps because it is intended to prepare students to be able to more easily interpret publications that tend to be in English as a lingua franca?

Do you and all the other students speak that native language? If yes, it's the English part that is awkward.

@Cape Code: Though I recall one graduate physics course (in an American university) where the instructor and students would often converse in Chinese, because I was the only native English speaker.

It might be helpful to know why the course is taught in English, when all the students (and teachers?) know the native language, and are (one would guess) more proficient in it than in English. It has happened that courses in non-English-speaking countries have been given in English for reasons that are more cosmetic and political than anything that actually benefits the students. ("Look, our small, unknown university is international!)

Did students have the opportunity to choose between English and local language courses or was English the only option that was forced upon them?

@Agent The medium of the degree is English.

@Peaceful If there aren't any students who are left out by understanding the question, what negative outcome are you concerned with by his method of communication?

What exactly is the point in teaching in everyones second language?

@NeilMeyer Presumably so the students don't need to relearn the material in English when they are encountering it again out in the "real world" outside the course.

It may surprise people that being both proficient and speaking the local language may be an in demand skill

How good is your own English ? If you aren't (near) native level, this might play a role.

Honestly, when my whole class was composed by only native speaking students I switched to a reasonable mix of native language + technical English words myself. I view the language as a mean for communication, not the end purpose of the lecture. I saw this made students more comfortable and more willing to participate too, as it removed one layer of unnecessary complexity for them.

Almost all science courses in our country are taught in English. Thus the medium of communication is be default English during the lectures.