Update GUI Translation demo for 4.0

[Calinou]

• Add PO support in addition to CSV.

Note that this PR does not include support for CSV plurals, as that feature isn't merged in Godot yet.

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• This salvages the work done in https://github.com/godotengine/godot-demo-projects/pull/516. Thanks @SkyLucilfer for the original PR :slightly_smiling_face:

[Alex2782]

https://github.com/godotengine/godot-demo-projects/pull/974#pullrequestreview-1686715380

Also, do we need to ship all the font with the demos? For the Mirroring and Translation demos system font fallback should be fine, neither require specific font dimensions to work.

I wanted to check if the 'fonts' can be removed here too, but it already says in the description that they are necessary on some systems.

@Calinou this PR works for you? And master branch not? https://github.com/godotengine/godot-demo-projects/pull/974#pullrequestreview-1683694780

• Label HELLO_TEXT has here normal 'droid_sans.tres' font
• Label HELLO_TEXT has in master no font (empty) and this errors (but I have no problems with the Japanese font.)

  Class 'Font' or its base class cannot be instantiated.
  editor/plugins/editor_preview_plugins.cpp:803 - Condition "sampled_font.is_null()" is true. Returning: Ref<Texture2D>()

this PR