It should be very simple. The Basques, the Catalans, the Bretagnes, the Occitans: all are distinct peoples with their own language, so of course they're entitled to their own nation-state. And for that matter, the Welsh and the Scots too.
Any attempt to obstruct and resist this, and you will be drummed out of the ranks of the world's civilized industrialized nations, and dumped in the bin of the Axis of Evil, along with places like China that still hangs on to Tibet and Uighuristan.
Of course, then comes the more difficult part, since there will be some practical difficulties in returning New Zealand, Australia, North, Central, and South America all to their indigenous peoples. It may take a few decades to phase in the needed relocations for that. But independence for peoples that are already the majority where they are located presumably won't involve massive relocations.