a good framebuilder is probably going to have a long waiting list ...
in France Jean-Pierre Levacon http://www.levacon.com/
is located in Marcilly-sur-Seine , 300 kilometers from Luxembourg.
He works with his wife and they made all sort of frames ( mostly Ti and steel) with great craftmanship.
That's an old school guy in his 60's working very well, but you will have to wait 1 year at least and I don't think he speaks english.
I can't think of any other builder in north or east France near Luxembourg.
François Kerautret http://www.fkc-concept.com/
would be well worth considering , he's probably the only one in the world making frames with his own solutions ( rear stays deported on the right to have a symetrical rear wheel, no rake fork, ... ). He comes from the motorbike world with another point a view.
Seems like everyone who has tried or own one FKC says it's the best bike they know.
but he's in the south of France.