You're asking two questions here: administrative borders related to territorial disputes, and map label localization.
Since you mention OpenStreetMap, I feel obliged to answer:
The OpenStreetMap Foundation changed the policy regarding territorial disputes in 2013. You can find the full document here, but let me quote the relevant paragraph:
Borders and boundaries
National borders are particularly sensitive. Currently, we record one set that, in OpenStreetMap contributor opinion, is most widely internationally recognised and best meets realities on the ground, generally meaning physical control. In areas without clearly defined borders, the line is approximate. Our database structure enables mapmakers to easily ignore this set and substitute another more appropriate to your needs.
In the future, we may look at supporting alternative sets directly.
In other words: if there's a physical wall, then OSM records that wall, and not the theoretical line(s) encompassing the territory that each country (or govt/administrative entity) claims as their own.
You will not find the border lines of Mandatory Palestine, nor the border lines of the territories claimed by the State of Palestine in OpenStreetMap. Conversely, you will also not find the border lines of the territories claimed by the State of Israel.
OSM's sister project OpenHistoricalMap might help to provide historical borders, as well as the Humanitarian OpenStreetMap Team, which did some work in the West Bank in the past.
Localization of the map's text labels in OSM is mostly a technical problem. In a nutshell, it involves rendering your own tiles but choose from a localized
name tag instead of the default.
You'll find technical information on the OSM wiki's page about map internationalization , and specifically, on the design documents for the Multilingual maps Wikimedia project.
You should consider not using OSM for this task. Keep in mind that Leaflet is agnostic in regards to the map tiles, so you can use any data source (i.e. non-OSM). In particular, be on the lookout for WMS services, as well as for shapefiles/geopackages with relevant data. Using a non-OSM dataset placenames might be easier for your use case.