I've always wondered, why is there such a massive difference in popularity of this app in and outside of Czechia? Do you think it's mainly - lack of focus on non-Czech user experience (poor translations, map layer looking worse, ...), lack of marketing, something else?

I guess it's a combination, the lack of marketing for sure. Perhaps it's also the algorithms fault, if only a few people (considered from a worldwide view) download it, it's not shown to others looking for map apps (it's a very crowded space app wise). Czechs are very outdoorsy people, they recommend this app to each other and so it's very popular there, but unfortunately the language barrier makes it harder spreading over borders. But these are all just wild guesses, who knows? Most people here in Germany use Komoot which gives away their maps for free on a regular basis, perhaps that's better marketing than "everything is free already"... :)

1. Mapy.cz is not marketed outside of Czechoslovakia.

2. It has its own (high quality) map data only for Czechoslovakia, outside it uses "just" OSM, although you'll get arguably superior rendering there.

Czechoslovakia is not a country and hasn’t been for 28 years.

Mapy.cz uses OSM data worldwide except Czechia, where they have their own dataset.