You're describing a blockchain with nation-level votes instead of proof of work. I don't see any valuable properties in such a system compared to a normal bank. If the member nations get to control the contents of the blockchain anyway (because they can locally decide to to sybil attacks or whatever), why would I bother using such a system instead of just having an account at Barclays or whatever?
Barclays doesn't require the consensus of 51% of countries to do shenanigans. Plus their shenanigans are easy to keep hidden for years, whereas this attack would be quite obvious as it was performed.
Yes, both require some level of trust in governments, but that doesn't mean they are the same.
