Could indexation of wages help Argentina?
It seems that you answered your own question?

If the government's estimation of inflation rates are not credible, how can the government effectively index to inflation?

In any case, the first best solution to inflation in the long run is almost entirely an independent, competent central bank that has the capacity to raise interest rates, limit the money supply, and bring inflation under control. (And have a government that doesn't interfere with the central bank.)