French flag flying, start European (most likely culprit), unless you live in Europe, it probably was a WWI or WWII bringback
Edit: The flag did change yes, but the artist has creative powers and MAY have put that in.
Edit 2: I can't find it anywhere, 1800's or 1900's oil paintings of a sailboat flying French colors, BUT the style that is was done in was wildly popular during those times.
Edit 3: Like some folks above said, go get it appraised. I would hang it if I had it, but it's good to get it checked out. Lets hope it wasn't stolen by the Nazi's and resold unbeknownst to the buyer.