My feeling would be that for a Bachelor's degree, you have a very limited audience - essentially your tutors. Not much different from a Master's. But I'd have thought that for original research – the point of a PhD – you'd want your research to become widely read, and that if you were bilingual, with a widely shared language as one of your languages, choosing a language spoken by 0.01% of the world's population would be limiting, compared with one spoken by 18%. (Gemini)
But maybe not writing in your native language would be 'selling out'. I dunno. But I suspect that most international scientific conferences are likely to tend towards English as the lingua franca, for the most efficient sharing of ideas.
