This kind of decision depends on a referendum.
Up to now, the french speaking Swiss were largely in favor of joining the EU, but not the german speaking Swiss who are a majority.
Once, in a trip to the US, in the bus there were several french speaking Swiss and also german speaking Swiss. They practically never spoke to each other as they didn't know the other's language. And the little Belgian could be understood by both sides. Switserland is a confederation with "cantons' that have hardly anything to do with each other, seemingly. It's getting like this in Belgium too: for instance, TV weekly magazines in one language don't bother with what happens on the other side of the linguistic divide.

Raoul