Write a computer science thesis in native language or English?
As was suggested in the comments, if you want to share your thesis with researchers in other countries, it is better to write it in English than in other languages.

However, I will go further than that and say that in the 21st century, English is the lingua franca of communication in most technical fields. If you want to participate in the wider technical community in a meaningful way, developing your ability to express yourself in English, both orally and in written form, is essential. Therefore, you should avail yourself of opportunities to work in English whenever practical. 

It is still important to be able to communicate in your local language, but this is becoming less of an issue as time goes on: for many people, they do scientific work in English, even if their everyday workplace conversations are in another language!