If you visit Luxembourg City and enter one of the shops, you may notice that the Luxembourg shopkeepers instantly switch back and forth between several languages as they wait on the foreign tourists. It is quite usual for a native Luxembourger to be fluent in four or five languages. Meanwhile, you may hear them conversing among themselves in a strange German-sounding dialect.
Every Luxembourg child learns German and French beginning in elementary school, but most likely speaks Letzebuergesch as his native tongue at home and while playing with schoolmates. That is why all Luxembourgers are at least trilingual. Many also go on to learn some English, Dutch, Spanish or Italian as well.
Letzebuergesch is a distinct language with Germanic origins that date back at least 1000 years. It has differentiated enough that most German speaking peoples no longer understand it. Technically, it is classified as a West Moselle Frankish dialect and is closely related to various dialects spoken in the neighboring regions of Germany, France and Belgium. Here is a link to an interesting web site by Roger Thijs discussing these various dialects.