Perhaps another way to find untranslated non-fiction books of interest in languages I cannot read is to not search directly for them, but to find authors first.
Scientists, practitioners, and thinkers of various disciplines may be mentioned in localised Wikipedia versions, and from there I might find their bibliography.
In the past few days I’ve been exploring the non-fiction section of online bookstores across the EU a bit, but then books translated from English to the other European language easily make it harder to see other publications.
I was mailed two book suggestions in Swedish. The authors have Wikipedia pages, i.e. Julia Ravanis and Peter Gärdenfors (who also has an English wikipedia page btw), and those pages list their bibliographies.
Going this path makes it possible to use the well known Wikipedia structures to search for authors across languages.
What it lacks is networked recommendation.