Add translation preview in editor

[timothyqiu]

Follow-up to #96230
Closes godotengine/godot-proposals#8171

Allow previewing the edited scene in the specified locale.

Mainly a proof-of-concept for now, marking as draft. Not thoroughly tested.

Peek.2024-09-13.00-48.mp4

[AdriaandeJongh]

Incredible work, truly.

One UX note that wasn't covered in the proposal: when a translation is selected, there is a discrepancy between what is in a Label's inspector and what is shown in the scene. Perhaps some sort of indication in the text field of the fact that the field is translated is in order?

For example: the text field is expanded to include a row explaining what this label translates to (ugly example incoming, but you get the point...):

[Calinou]

Tested locally, it works as expected (including with custom language codes).

As @AdriaandeJongh pointed out, I agree we should have some kind of visible notice that the translation is currently being overridden in the editor. This could be an overlay displayed in a corner of the 2D editor viewport, so that you don't need to select a specific node to view it. It could be shown below the zoom level, with a globe icon and the language name next to it.

Some more feedback:

• Language codes in the dropdown should display the language names when available, like in the project manager dropdown.
  • Custom language codes don't have a language name set (not without Update language list, add new locale selection dialog and support for custom codes. #103498 at least), so only the code should be displayed if we try to get the name and it's empty.
• I suggest keeping the dropdown open after selecting a language, so you can switch between languages more quickly. Call set_hide_on_item_selection(false) on the submenu's PopupMenu to achieve this.