I think part of it is that - in Western fandoms - all the canon material is in English. Switching to thinking about it in my native language is weird, to say the least. And then all my friends are from abroad, so I use English to both consume canon and talk about it - and it is kind of a specific dialect, in a way? Translating it is more effor than it's worth and it doesn't come natural to me. And that's not starting on how tiny non-English parts of fandom are.

(I also can't talk about comp sci in anything that's not English, because the vocab just doesn't exist :P You end up copying English words anyway. Similar issue.)