I speak both English and Spanish natively, however, I'm more comfortable in English when talking about e.g. technology and music. On the other hand, I'm more comfortable in Spanish when talking about math, feelings, etc.
As you can probably already tell, it's not like math and feelings are very related. It's just that I've probably been more exposed to those things in Spanish than in English. Same goes for technology and music topics, I've just been exposed to them in English a lot more than in Spanish.
Btw, I also had to write a dissertation in Spanish and I, too, had trouble with it, hah.
From memory from around 1850 up to 1920s.
(I'm not that old heh, but I know people who have done serious historical research in engineering that required German)
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…