OpenHistoricalMap labels should be in the preferred language

[1ec5]

OpenHistoricalMap is currently shown with the labels in the default language, which is intended to be the local contemporary language. Perhaps users would prefer to see the labels in a consistent language matching either the frontend language settings or the server language, as requested in this forum topic.

Since OHM is being consumed in vector tiles, it’s possible to swap in other languages on the fly, as long as OHM has those alternative names. The mapbox-gl-language plugin can systematically swap label languages throughout a stylesheet. It’s a bit finicky and I’d like to write a half-decent replacement, but it should do for now. OpenHistoricalMap/openhistoricalmap-embed#8 demonstrates language switching based on a URL parameter.