The right spelling in english is MARTHA. Check Wikipedia and other english sites
Oh yes, of course, I do know that. I was wondering what the official English forum spelling of her name would be.
Should we really call her Crown Princess Martha? After all, we do write "Louis-Philippe" (King of the French) and not "Lewis Philip", so there are limits to Anglification.
I don't know how others feel, but for me using the correct diacritics and spelling in the names and titles of royals is very much akin to giving them their due styles. There are differences between Imperial Majesty, Imperial Highness and mere Highness that you just can't ignore if you want to treat royalty by its own, cosmopolitan terms.