SUMMARY Use user's selected language for toponyms. Not English nor vernacular languages. For example: Warsaw is the english name for the capital of Poland, which vernacular name is Warszawa, and the name that appears in Marble even if your desktop is in other language. But for the rest of the world both names don't mean much: in italian is "Varsavia", in spanish "Varsovia", in russian "Варшава", in german "Warschau", and in non latinized areas most of us can't even read it because it's written like "ਵਾਰਸਾ", "وارسو" or any of the many alphabets that people use in their respective countries. If you know Android's app Osmand, it has an option for this, so it's not a problem with Openstreetmap but a Marble issue.
Agh! These comments should be editable for soem minutes after posting... When I wrote about not having the texts in MArble in english" I obviousle was thinking of the majority of people, which language is other than english; didn't mean that north americans, etc, should not have their texts in Marble in english, of course.
BTW, the link for the mobile versión in https://marble.kde.org/ points to the same than the desktop version.