I18n

Hello, that pull request fixes issues and adds improvements of #2710.

@REAS and @benfry: Could you please label "language" issues with a separate label, so I can find and validate these issues. Thank you! And we should be careful with new structures within the .properties files, because if we merge everything, it could end in spaghetti structure.

Happy coding,
Darius
Regarding the sketchbook frame, I would prefer the use of `sketchbook.window.title` rather than `sketchbook`, as it is more clear for the translators.