Support multi-language text document comments

[Inotart]

As is well known, in Godot, variables can be annotated and can be made visible in the editor, for example:

## 这是一个名为a的数字
@export var a:int =0

Then is there any simple way to support multiple languages for annotations at once?
I just make a random assumption here, but in reality, I guess this won't be done.

## This is a number named "a".
##
## You can see this
## zh_CN: 这是一个名为a的数字
##
## 你可以看见这个
@export var a:int =0

The default language is English. The comment shows:This is a number named "a". You can see this
If the editor is currently set to Chinese, the following message will be displayed: 这是一个名为a的数字 你可以看见这个

This proposal should also be applicable to the documentation comments of GDExtension and other similar functions.

[dalexeev]

In my opinion, if you want to support several languages ​​for custom documentation, then duplicating descriptions in doc comments is not a good idea. Because it would take up a lot of space in the source code and make it difficult to synchronize translations.

I think it would be more preferable to have documentation in one language, and translate the rest of the languages ​​using Weblate or other software. This can already work for online documentation (.gd -> .xml -> .rst), the only question left is how to display the translated documentation in the built-in Editor Help.

We should probably add the ability to load translations of user documentation into the editor via a plugin or via files like lang_code.gddoc.po.