For my PhD thesis, I was required to write an abstract in my native language, German, and had a pretty hard time of it. On the other hand, I've experimented a bit with high school level CS teaching, and found that with sufficient preparation, it's fairly doable.

I suspect that when we struggle to express a concept in our native language, we may have somewhat deceived ourselves in how clearly we expressed the concept in English. I notice all the time that people think they are more profound in a foreign language, when it's in fact precisely their lack of familiarity with that language that disguises the banality (A phenomenon also known as "quidquid Latine dictum sit altum videtur").

Somewhat related, I see people cursing in foreign languages…