Luxembourg can be very confusing for foreigners. It has three official languages: French, German, and Luxembourgish. There's also a fourth semi-official language: English. While many speak all of these languages, it can be a challenge to figure out which language is used when. I stumbled across this article, which may be useful in straightening out some of the confusion.
Of course, the article describes the situation in modern Luxembourg. Many Americans date their connection to the Luxembourg to the 19th century. From what I've read, French was less common and German far more popular than it is today. So how do you figure out what your ancestor spoke? Very carefully. Look at all the available information.
For example, my ancestor is remembered as speaking French. His home village is the only one in the area that has regular services in French. But was French his primary language? Probably not. Luxembourgish has words that could easily be mistaken for French. My ancestor married a German woman who wouldn't have spoken a word of French. Based on the best evidence available, he probably spoke German, Luxembourgish and English - but I can't rule out the French.