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
The title of the sketchbook frame is defined in `showSketchbookFrame`. This is only used to populate the Sketchbook menu item in the default File menu for OS X.
````
app/src/processing/app/platform/ThinkDifferent.java
137:    fileMenu.add(base.getSketchbookMenu());
````