One of the most important outcomes of the First World War was that it led to the central powers all breaking up into nation states while Russia lost its western territories to new republics in Finland and the Baltics after the Russian civil war.
Thus all of Europe east of the Rhine was henceforth made up of independent 'mother tongue republics' while Ireland gained independence from Britain.
Have this happen not just out of the central powers and Western Russia, but also France and Britain, so that Wales, Brittany, the Basque Country and Occitania all become independent. Bonus points if Scotland does too.
What happens to colonial empires?