You could ask at the paint counter at OBI, if no one here can help you. My sister in the US used a citrus varnish stripper last summer that was non-toxic to re-finish her coffee table. But it was a LOT of work because she had to scrape the old varnish off for hours after applying the stripper, and even though it was non-toxic, the fumes were still pretty nasty.
Have you considered using a sander instead? I’m planning to re-finish a dining table at some point and was thinking about getting this one, which is surprisingly quite cheap, because it seems like it would be a lot faster and with no nasty fumes or chemicals. You could probably also rent a sander for very cheap on a site like sharely.ch (a tool-sharing website):
Bosch multi-sander
https://www.amazon.de/-/en/gp/produc…KR8XB7XF&psc=1