It's simple. Just like learning an instrument is seen as amazing, learning a useful language is seen as admirable as well. Imagine, a person you never expected plays the violin adequately, you subconsciously start to think of them as talented. And learning Physics would do the same. It's just harder to show Physics aptitude than it is to show knowing a second language. How many stories of people being low class, but then solving a hard math/physics problem and changing their lives? Or how about this: The MC is sleeping or shows up late for class, the teacher calls them out and says solve this question on the board. And they solve a problem most of the others could not. That's a classic way of showing competence. in story.
Also, learning a second language when you don't use it every day is even more of an achievement. So sure, she may be a teacher and not use it every day, but what that shows people is that she's smart enough to learn a second language easily.
