So, how do you say "You saying so doesn't make it so" in Latin? I think it would be a literal translation of Croatian "Tvoje to reći to ne čini", Tuum id dicere id non facit, but I am not sure.
I'd suggest a non-literal translation to capture the idea:
Quamvis dicas, non fit.
However much you say it, it does not happen.
Or even more pithily:
Dicas, non fit.
You may say so, but it does not happen.
The point of these approaches is to convert "you saying so" to a concessive clause, which strikes me as something idiomatic for this purpose. Brevity also gives the message strength. Concessive clauses can be introduced with or without a conjunction; see the two grammar links for details.
If the conclusion is not about happening but being, then replace fit with est.