I have published a scientific book (i.e. a text book) alongside with my supervisor (he is an associate professor, by the way). Since the book is not in English and consequently does not target international audiences, can it provide some remarkable value in my CV for a PhD program abroad?
I don't think the language matters. It is a substantial achievement, I think. If it is in your field, which I assume, it will have value. But, depending on where you are applying, you will need other things as well. If your contribution was "only" providing exercises, then it might be a bit less, but for content contributions it would be good.
But no one thing is determinative of your suitability for admissions.