Your first setup

A server in one language

Set language.force = "tr" in config.conf and /uxmlang reload. Everybody reads Turkish, whatever they chose and whatever their client says. /lang set answers that the server reads one language. You may also take uxmlang.use from everyone so /lang is refused:

lp group default permission set uxmlang.use false

Players choose, nothing follows the client

Set language.follow-client = false and keep language.default = "en". A new player reads English until they pick a language with /lang.

A network

1. Point storage of every server at one MariaDB, MySQL or PostgreSQL database, with the same table-prefix.
2. For changes that reach a server at once, set network.transport = "redis" and the same network.redis block on every server.
3. Restart each server.

A player who picks Turkish on the lobby arrives on survival reading Turkish.

Check it

Join as a player who is not an operator, run /lang, click a flag. The reply comes in the new language, and every UXM plugin's commands and windows follow.

Related: Concepts, A network