I cant tell the diffrence between a french and german person?

I have been married to my wife for about 2.5 years and I have always thought that she was german. She has never spoken french and has given me no reason to think she was french, until recently, when we were visiting her grandparents. While there they would speak french and my wife would respond in french and I was shocked. She isnt mad at me, but I still feel wierd about it.