Christian, feel free to correct any of this you disagree with, I'm not sure quite how much of this I remember accurately:
Lëtzebuergesch is of a weird status. It is a national language but is yet, according to the theories of Abstand, not a fully-developed language (more a half-language). It is developed from Rhine-Franconian dialects of German, but Luxembourg's independent status and distinctness from the rest of Germany (plus its official ties to French) have led to the dialect becoming usable for all functions of society. It now has, after about fifteen attempts, a standardised orthography (though not an enforced one) and is in the process of codifying its grammar, at which point it should be considered a language in its own right.
Lëtzebuergesch became an official language in 1984, I believe, or rather a national language, not an official language. French and German remain the main working languages of the country, with French holding the higher prestige, partially due to urban educated classes (much like in Brussels) preferring it, and partially due to some residual resentment against the Germans for historical reasons.