Upvote:1
Both Japan and China have long had people of different religious convictions (Buddhism and Shintoism in Japan, Buddhism and Daoism in China). But neither of these religions insists on "You shall have no other gods beside me", so they do not lead to the emergence of clearly distinguishable groups and therefore the effect on political stability is small.
Edit: That does not mean that these countries did not have large religion-related conflicts: Japan had christian uprisings and persecution of Christians in the 17th century, and China had the Taiping rebellion and several muslim rebellions in the 19th century. However, there does not seem to have been much conflict between the mainstream religions, i.e. Buddhists and Daoists and Shintoists.
Upvote:8
Switzerland consists of German-speaking and French-speaking populations, plus some smaller groups. Pretty stable for 500 years.
Upvote:13
Most empires (Achaemenid, Roman, Eastern Roman, Osman, Austro-Hungarian and Russian) had very diverse populations and were quite sustainable. Even if you count Eastern Roman empire separately from the Western one, the Western one existed for more than 400 years, and the Eastern one for 1000 years after that.
Actually many European countries have very diverse populations, and the question is really what is meant by "sustainable". How long should a country exist to be qualified as "sustainable"?