Add support for custom locales in the translation server

[GameDungeon]

Describe the project you are working on

An open-source evolution-focused video game.

Describe the problem or limitation you are having in your project

We have some users wishing to translate the game into languages that are not on the supported locales list. Currently, it is impossible to use unsupported languages with the current translation server setup in godot.

Additionally, this setup stops us from using non-conventional languages, for example, Minecraft's pirate language or Wikipedia's simplified English.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Adding the ability to register custom languages into the translation server would resolve these issues.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Either the languages will be registered into the translation server with code at runtime, or they will be registered using configuration files, whichever is determined to be easier to use / implement.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, I don't believe it can be worked around.

Is there a reason why this should be core and not an add-on in the asset library?

I don't think this is possible to to implement as a plugin, but even if it could be, stuff to do with language handling should remain core.

[envyniv]

Also having this feature on the 3.x branch would be a godsend, due to the even less flexible support for languages in the versions. 🙏 This needs to happen!