I'm looking for non-fiction books written in Non-English and non-Italian language that talk about conspiracy theories.

A number of your questions like this one would be better served by a research librarian in a large library. If you are in a city, go to a large library and ask for the research librarian. They will have access to tools that we don’t have and are getting paid to do this kind of work.

@luigirovatti this is probably not the best place to look for non-English books. I would guess that 99.98% of the people here know only English.

The research librarian suggestion is excellent (you may also simply be able to use a library index for this, and a librarian could help you do that):

And/or, check out the authors under:

https://en.wikipedia.org/wiki/Category:German_conspiracy_theorists
https://en.wikipedia.org/wiki/Category:Polish_conspiracy_theorists
https://en.wikipedia.org/wiki/Category:Belgian_conspiracy_theorists
etc…

@elbanditoroso No, I know . . . Italian . . . pretty well. (heh)

Written in other languages, or specifically written by authors outside of English speaking countries?