He doesn't need to take them to court. He negotiates a transfer as a free agent. Then up to Barcelona to prove he was not a free agent.
City are best keeping out until Messi v Barca dispute is clear cut. Once it is, we can submit an offer which would go through FIFA's arbitration service and La Liga can say what they like.
Barca either have to win Messi around, or they have to come to terms with his decision to leave. It's a dispute between club and player for now but City may decide that we have a opportunity to buy him if he has become a free agent.
It is evident that Messi regards himself as a free agent so I think the way forward is opening for City to make an offer. Messi can alternatively accept an offer from Barcelona and set this aside. I don't think he will but it will become clear soon enough. I don't think it will drag on for much longer.