After doing some research and testing its search features today, I found the Worldcat database to be the best candidate for multi-language ISBN search. It has more in that it also has other media, such as DVD's and audio CD's, but for books it gets correct results every time, even for 1980 books not printed anymore. All media included, they say its database contains more than 2 billion items.
It has also books in many languages including English, French, German, Japanese, Chinese, Arabic, etc.
Worldcat seems to be owned by the OCLC, an organism I didn't know existed, and which is according to Wikipedia:
"a nonprofit, membership, computer library service and research
organization dedicated to the public purposes of furthering access to
the world’s information and reducing information costs".
It is also indicated that they (emphasis is mine):
produce and maintain WorldCat, the largest online public access
catalog (OPAC) in the world.
... which by the way falls I think into the Open Data SE website.