Make editor language setting default to Auto

[timothyqiu]

Currently, interface/editor/editor_language defaults to the system language. This creates the problem that it's technically impossible to pin a language, as the setting is not saved when being same as the default value.

You can use the following steps to reproduce the problem.

Warning

Don't use Project Manager's quick settings dialog when testing the problem. It somehow saves all properties it sees on EditorSettings, including those that are not meant to be saved :(

1. Run Godot with LANG=es for the first time
   • It displays in Spanish, which is expected
   • Now I want to always run the editor in French, I set the editor setting to fr
     • interface/editor/editor_language = "fr" setting is saved.
2. Run Godot with LANG=es again
   • It displays French, which is expected
3. Run Godot with LANG=fr
   • It displays French, which is expected
4. Run Godot with LANG=es again
   • It displays in Spanish, unexpected
     • interface/editor/editor_language = "fr" setting is removed in the third run

This PR adds a dedicated Auto option as the default value.

Tool scripts can use EditorInterface.get_editor_language() to get the effective language instead of reading interface/editor/editor_language.

[bruvzg]

Code looks good, having a dedicated "auto" make sense for me.

[Mickeon]

There's one ramification: this needs to be documented in the migration guide. It technically breaks compatibility because interface/editor/editor_language is no longer reliable.