The map tiles published by OpenStreetMap at tile.openstreetmap.org are monolingual - they are images after all so there is no way to change the names on the fly and we don’t have the resources to host hundreds of different tile sets for different languages.
That language is not English though, rather it is the “primary” name for each object (the value of the name tag, ignoring any localised names like name:en or name:fr) so you should see English names in English speaking countries and French names in French speaking countries.
We are working on moving to vector tiles which opens up the possibility of supporting localisation by using separate tile sets for for names but I can’t guarantee that will definitely happen or when exactly it might happen.
I’m not in front of a machine with a desktop client that I can try right now but I believe there is a selection of maps available there? In which case it will depend on which one you are using.
The easy way to tell is to look at the same area on https://www.openstreetmap.org/ and see if you see the same names, and the same map style.
Gramps Web v24.7 also introduced OpenHistoricalMap as a new layer, which uses vector tiles (high res!) and displays time-dependent place names in the local language:
So it not a bug nor a bad setting, only due to Carto Voyager. I’ll see later for Leaflet layer.
For instance my main priority is to learn more about “docker” and I’ll probably have to deploy a reverse proxy… but that’s another story
I tried OpenHistoricalMap looks really useful, but lot of town names are missing for me yet.