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!

If your english isn't very good and it would be a considerable hardship to write it entirely in English then there is a third option:

Write the thesis in your native language but write an extended abstract in English.

By extended abstract, I mean a more detailed summation of your key findings than is usually provided in the short (250-500 word) abstract that prepends dissertations.

Quality is better than quantity. I would prefer having a short 3-page well-written English summation that has been meticulously written and proofread than 300 pages of poorly written English.

Pros of writing in English

• You can share your work with non-Dutch-speaking people. Especially important for graduate school applications, since after that unless you did something really brilliant nobody will care much about your undergrad work.
• It gives you some experience in English-writing, which it seems you need (since you are still somewhat uncomfortable with it). There is no denying that, like it or not, you will probably need to express yourself in English a lot in whatever your future career is. You want to do it well, beause working in English with people who are bad at it is a real pain.

Pros of writing in Dutch

• Avoids having your point obscured by English mistakes, though based on this question it doesn't seem you would do more English mistakes than can be found in the average paper.
• Less effort, but I would argue that the fact that it is less effort to write in Dutch is an argument for writing in English. It's something you want to fix.
• You may (the question does not make this clear) simply prefer Dutch. Even though I am equally comfortable writing in English or in my native French, I still vastly prefer French, simply because I think it's a nicer language. (Yes, I may be biased.)

All in all, at this point I agree with aeismail that you should probably write in English. However, due to the point immediately above, I think his answer ceases to apply if and when you have attained a satisfactory level in English expression. Then, you shouldn't feel any need or obligation to use English "whenever practical", just because mostly everyone else does it. In fact, I think I do the opposite: I use French whenever practical, and only use another language when French doesn't work.