I am currently writing a fiction novel. I have about 5 finished chapters. But, since English is not my native language, (it's actually my fourth language) I feel the urgent necessity of sending my work to someone abroad to give me some feedback (a native speaker, not that I don't trust him). However, I fear that my ideas might be copied or stolen from me if I keep sending more chapters. What should I do?
It is not clear from your question what the source of the risk is.
If the risk is that your beta reader(s) (in another country) will convert the material for their own use, you could enter into a formal contract with them. That may not provide much relief in as much as the claim would be international in nature. The only thing that you can do is to pick trustworthy beta readers. You might want to look for those beta readers in your own country. Perhaps there is someone who has moved to your country from the "old" country. My language is awkward here because I do not know the countries involved.
If the risk is that your writings might be intercepted by a third party, there are ways to encrypt the files such that only the receiver(s) can see them. The specifics of that would depend upon the technological capabilities of the receiver and the legal restrictions on the use of encryption in the receiving country. This assumes that the beta reader(s) will exercise care in the handling of the chapters. That is a substantial obligation to impose on anyone.
There are ways to establish that you wrote the materials on a certain date, and that any use of those materials after that date are illegitimate. Research copyright to understand what can be done. Again, pursuing legal action in such cases, even if it does not cross national boundaries, is not at all easy or productive.
Finally, your ideas are not something that you can easily protect through the law. The expression of those ideas, in writing or in other media, can be protected. But not easily.
The answer to this question very much depends on your jurisdiction and what kind of artistic theft you intend to prevent. In terms of protecting your writings themselves from being plagiarized by the native English speaker, there are a variety of ways of proving you wrote the text first. In terms of the native English speaker "stealing your idea" whatever that might mean, that's a risk you take when you share your work with anyone.