Who decides if a country is sovereign or independant?

War, usually.  Revolution sometimes.

On International Diplomacy, a country only is considered sovereign (a
National State) when most of other countries recognise it.
Think about our present questions about it. Palestine State does not
exist because the International Community because the international
community does not recognise it. In case of USA, the Declaration of
Independence was signed in 1776, then started the War of Independence.
The real Independence just happened when the metropolis has desisted
from colony, in 1983.
E.G.: In 1948, the Israel State was created, because an international
accord was signed by all "countries" of "just-created" United Nations.