What is right: SPACE or NO-BREAK SPACE ? Does this make any difference?

I would suggest you use a regular space, since a menu item can’t word-wrap onto a new line.

It *might* make a difference if the font specifies a different width for the non-breaking space than a regular space. Since this is not a typeset environment, you can’t rely on a layout manager to reassign values to space characters, so you want all the spaces to be the same width. Hence use the same character. (In practice, there’s likely no noticeable difference.)

Another problem is missing context in MainMenu.strings:

But the only context given is:
/* Class = "NSMenuItem"; title = "Cut"; ObjectID = "uRl-iY-unG"; */

It was reported in the developer forums recently that the localization comment field that you can enter in IB (the very last field of the Identity inspector) isn’t being output for Mac targets, though it is for iOS. I don’t know if this is fixed in Xcode 9.3. If it does work, this is where you can specify the context.