A government doesn't declare things "illegal", judges are the ones that do it. According to our constitution, Spain is not divisible, plus regions can't decide

A government doesn't declare things "illegal", judges are the ones that do it. According to our constitution, Spain is not divisible, plus regions can't decide something like that in a referendum because national sovereignty belongs to all Spanish citizens. I.e. regions aren't sovereign. Same as most if not all federal governments out there, by the way.