Luxembourgish, a language unique to Luxembourg, is most closely related to which other language?

German and French are the most spoken

and aprox ... 390,000 people worldwide speak Luxembourgish.

Luxembourgish is also spoken in small parts of the surrounding countries of Belgium (in the Province of Luxembourg near Arlon), France (in small parts of the Lorraine) and Germany (around Bitburg and Trier). In Germany and Lorraine it is simply considered the local German dialect. Since the Second World War, however, the language has not been taught in these countries, with the result that use of Luxembourgish is largely restricted to the older generations.

Furthermore, the language is spoken by a few descendants of Luxembourg immigrants in the United States, and a closely related variety is spoken by ethnic Germans long settled in Transylvania, Romania (Siebenbürgen).