/not a teacher, just want to share some tips with you.
I don't think it's necessary to study the subject first in your native language. It may help you a lot but also consume your time. I'm studying electrical engineering in English with very little background.
Anyway, it could be better if you can tell your main problems. What are your difficulties? If you find it hard to remember the vocabulary, I suggest using a spaced repetition flashcard software like Anki or memrise.com (a website).
Anyway, do what works for you. If you find it better to study philosophy in English if you understand it first, then go for it.