What happens if we have a provider that is both locale- + bound-aware?

It has to implement two interfaces. You will run into an issue when you typehint for it:

function (LocaleAwareProvider $provider) {
  if ($provider instanceof BoundsAwareProvider) {
    // Set bounds
  }
}

I do not know a better way of solving this. To create Superinterface extends LocaleAwareProvider, BoundsAwareProvider is wrong... To include all methods in one interface would violate interface segregation principle.

Note that this is only an "issue" in third party libraries. An application developer would write:

function (ConcreateProvider $provider) {
    // Set bounds

}

This is the wrong approach. We should use GeocodeQuery::withBounds