Here's a possible clue; the bolding is mine: About half of San Marino citizens are residents abroad, mainly in Italy, the United States, and France.
This is from:
http://www.1upinfo.com/encyclopedia/S/SanMarin-people-economy-and-government.html
The site says a community there was formed by the mid-5th. century--rather before anything that relates very closely to modern-day French, is it not? There is no mention of ever having been ruled by France, so I kind of doubt that what you heard is a legacy. I dunno--I'm just stringing together a couple of facts with some rather tenuous logic. Um--I was ...surprised isn't quite strong enough...bemused, maybe--to read that they have a political party called the Christian Democrats. This is a mind-boggling oxymoron, to me.
(And, NO, I am not inviting discussion on that last, here, any more than I was in the other thread; simply posting some thoughts that are new to me. If anyone is offended, let me know and I'll be happy to delete.)