"Finland and Japan are pretty much homogeneous."
Attempting to describe any country as homogenous is not a very good idea. I (possibly) understand where you are coming from but even within notional national boundaries there are other identities. For example (somewhere I do know about) within the UKoGB&NI, there are at least four national identities (England, Northern Ireland, Scotland and Wales) then there are the other - relatively - nationalistic movements such as Cornwall (Kernow) and others that arise from time to time. You may or may not know or at least be dimly aware of Eire - the Republic of Ireland - which was once part of GB and gaining independence in the last century. Then as we slide back through time it gets increasingly complicated and involves bits of what we now know as France and Denmark with a light dusting of Sweden and a host of supporting characters. That's just the collection of islands to the right of mainland Europe. The history of the rest is a massively complicated web that has to be studied to be believed.
Notions of nationhood and ideas of some form of homogeneity just don't work. Apart from anything else, try and define what a nation really is. For example is Wales really a nation? Yes!! From one perspective it is part of England (I'm English). It is referred to as a "principality" and has a Prince of Wales (currently Prince Charles.) The Queen (Elizabeth), who is actually many queens eg Queen of Australia and and many other countries and in this case of Great Britain which includes Wales as part of England and Scotland. When we add in Northern Ireland we get the United Kingdom of GB and NI. Despite all that, Wales has a National Assembly ie local government and there is a national language which is fairly pretty widely spoken alongside English.
Japan and Finland both have a history that is rich and deep and bloody complicated. I know this purely because I know how the history of Europe works in some detail and I am gradually working my way out.
Parochialism is an easy trap to fall into. Please read and travel more: the world is an amazingly diverse place. You think the US is a bit mixed up? You have no idea mate. I'll give you three books to read for starters: "Germania" by Simon Winder, "1000 Years of Annoying the French" by Stephen Clarke and "Stalingrad" by Antony Beevor. All three are admittedly by British authors.
Anyway - there is no such thing as a homogenous country.