Saying that French is mandatory to live in Luxembourg is right but also wrong.
If it's the language of daily life in the centre, south and west of the country, with all adverts, shops written in French, you must to know that some people, above all very old (who were at school during the German Occupation when French was forbidden to study) and some who live in North landscapes and East, even in main towns, where the second language is German. Also, for technical jobs, and others, the German language is mandatory.
This antagonism is highlighted in education. Indeed, to continue in general, you must master German language, even if the studies language is French. If you don't speak German very well, you go in technical section.
You can have free Luxembourgish courses in your town.
Here is this, I wanted to talk you about this, because you are already lost with language questions... I think this post will go you away in depressiveness