I think this has more to do with budget. In many countries, professors are not paid specifically to write books, but if no proper book exists in the local language then they sort of have to do it one way or another. It usually ends up as some hodgepodge document, each chapter written by a different professor - whose are experts
at their fields, but not at book writing!, which is a specific skill. Some or better at it, others absolutely suck. Then they just print it some way without any lecturing, unification of styles, references and use it as the course book, because they were sorta forced to.
In English, there is competition and people with actual experience can publish books, which will be used by multiple universities, if it becomes famous than multiple generations of students from multiple universities and years have criticized it making the nth version better, etc.
This is very different from the budget solution my med university (of which 4 exists in this language alltogether) could reasonably come up with.
Nonetheless, there were smaller topics, documents, chapters which easily surpassed the same found in any English language book, especially in mathematics (the Soviet block used to be famously good at mathematics, so the level was much higher than the west’s), for these small gems it does worth speaking obscure languages.