"Do you want Catalonia to become an independent state in the form of a republic?"

That is a leading question by any measure. Try this:

“There is an opportunity for Catalonia either to remain a part of Spain or to become an independent state in the form of a republic. Please state your preference by ticking one of the two boxes below...”

Make no bones about it, if I were a Catalonian living in Barcelona I would campaign and vote for Catalan independence (when not at the Nou Camp). But either you do it properly or you just store up trouble for the future. It’s hard enough to avoid that as it is.