Contact the dojo ahead of time and arrange to observe class. Don't just show up out of the blue. When you go, introduce yourself, then sit where they ask you to sit and quietly watch the class. Don't speak unless spoken to, but someone will probably approach you to give you some information on the ryu and answer any questions. When you leave, thank the head instructor.
Otherwise, just act normal. It's a dojo in France, after all.
Josh Reyer
Swa sceal man don, žonne he ęt guše gengan ženceš longsumne lof, na ymb his lif cearaš. - The Beowulf Poet