In order to be able to have all languages available in all gameworlds we will need to change the structure of the gameworlds a bit by having all of them under the same domain.
The first idea we had, which is NOT the final idea, is to use the same approach other games like League of Legends have regarding servers. To have regional servers, where the region could be something like Europe, Asia, America (still having all languages available everywhere and servers on the same domain) .
i remember how not well travian can create regional servers in the past.
When my national server was merged into newly created regional server which operates multi-language, we were given mirror login pages to create account in national language environment. While logged in from national mirror, and having set the language to national, the welcome message was sent by travian in different language.
Being unable to read system messages without using external translator was one of the reasons making players quit. The player base shrinked notably in the region.
Unless you can make system messages match the selected UI language, do not add multiple languages!