First of all, asking about one thing to find out another is sneaky and somewhat manipulative. If/when people find out, they may feel hurt by this. Secondly, I like the idea of expanding a class to include another language, but it has to be done right, and if it's done without prior warning, I strongly disagree with it.
A lot of answers focus on the harm that can come to students who learn concepts and jargon in a language that isn't the predominate industry language. And that's a valid concern. On the flip side, sort of next to the 'favoritism' thing, but beyond perceptions and hurt feelings... It's unfair to the students who don't speak your native language to have parts of the class that they can't access.
I'm an American English speaker, and I'm now imagining being in a class with 30% hispanic students (which is accurate to how my elementary school was), and having the bilingual teacher teach certain math mnemonics only to those students who understand Spanish.
Granted, the reverse exists here, and is just as (or more) problematic, especially for children. But I agree with the other answers here: There's an understanding that the course is taught in a specific language, and the students signing up for the class are not expecting to need to know a different language.
If I were in your class and was one of the few students who did not know your native language, and you were to explain important concepts to only those who did, and I missed valuable information because of this, especially if my grade suffered or I felt like I couldn't apply the rest of the information because of the missing content, I would likely go to the dean or try to ask for a refund for the course. And although there may be no intentionally missing concepts, it's easy to forget that you haven't explained a concept, because you explained it on the side.
On the other hand, if it was in the course description, "Professor speaks X and Y", or "Office hours help available in language Y", then there's no problem with that. The students know what they're paying for before they sign up for your class. And the students don't have to tell you personal information to get to the point. To my previous example, if my parents had enrolled me in a bilingual English/Spanish school, no one could be upset if half of math class was inaccessible to me because I don't speak Spanish.