Who is the actor that plays the princess (or something) of Luxemburg in the 'Blackadder' episode with the Spanish bride in the first season?

If you are referring to the princess that he ends up marrying at the end of the episode, she is from Hungary and was played by Natasha King.

https://en.wikipedia.org/wiki/The_Queen_of_Spain's_Beard

This is the cast list of that episode:

Rowan Atkinson ... The Blackadder - Prince Edmund, Duke of Edinburgh
Brian Blessed ... King Richard IV of England
Robert East ... Harry, Prince of Wales
Elspet Gray ... The Queen

Tim McInnerny ... Percy
Tony Robinson ... Baldrick

Rest of cast listed alphabetically:

Jim Broadbent ... Don Speekingleesh, An Interpreter
Jane Freeman ... Mrs. Applebottom
Willoughby Goddard ... Archbishop
Harriet Keevil ... Lady on Ramparts
Natasha King ... Princess Leia of Hungary
Howard Lew Lewis ... Mr. Applebottom
Miriam Margolyes ... Infanta Maria Escalosa of Spain
Richard Mitchley ... 2nd Messenger
David Nunn ... 3rd & 4th Messengers
John Rapley ... Rex Lloyd
Stephen Tate ... Lord Chiswick
Kenn Wells ... 1st Messenger
Patrick Allen ... Narrator (voice) (uncredited)
Nicola Critcher ... Princess (uncredited)

Here is the link: http://imdb.com/title/tt0526542/fullcredits#cast