Catalan was dying out on its own long before Franco had anything to do with it. It was deliberately revived, as was Basque, early in the 20th century.
Franco squashed the 20th century trend to separatist nationalism, because he was a Spanish Nationalist, and because the separatists made deals with the desperate Republican government. It all came back after Franco.
One reason for separatism now is that the Spanish government, since it joined the EU, has been unable to shield the Catalans and Basques (their manufacturing industries) from foreign competition with tarriff barriers; that, plus the guarantee of a protected national market, was part of the unwritten deal keeping them in the union.
“Franco squashed the 20th century trend to separatist nationalism, because he was a Spanish Nationalist, and because the separatists made deals with the desperate Republican government”
Squashed, but not killed.