I hate to admit it, but for a long time, we weren't aware my ancestor spoke Luxembourgish. We knew he spoke German since he married into a local Bohemian family. We suspected he spoke French, since his children knew words of French. It wasn't until my first trip to Luxembourg that what we knew started to make sense. While he could have spoken French, the Luxembourgish dialect uses words of French. Luxembourgish was the most likely option.
Of course, realizing that has made me interested in learning Luxembourgish. My options are limited, but if you live in Wisconsin, you may be in luck. The Luxembourg-American Cultural Society offers onsite courses in Luxembourgish. Their contact information is available on their website. Even better, they're planning for an online course. I can't wait!