EU and China are perfect inversions of your point.
China suppressing ethnic, cultural, and linguistic diversity for homogeneity has caused immeasurable suffering and imperialistic destruction.
EU explicitly embracing ethnic, cultural, and linguistic diversity for heterogeneity has brought more peace and prosperity than ever before to a continent in perpetual conflict for nearly all of human history.
EU went to wars before not because of cultural differences though. It's not like Hitler necessarily wanted everyone to be a German, he just thought some cultures to be less important than others. Besides, you can always make the joke about the "paradox of tolerance" which by definition tolerates intolerance (which is probably at least one major ingredient of current rise of the right).
China suppressing ethnic, cultural, and linguistic diversity for homogeneity has caused immeasurable suffering and imperialistic destruction.
EU explicitly embracing ethnic, cultural, and linguistic diversity for heterogeneity has brought more peace and prosperity than ever before to a continent in perpetual conflict for nearly all of human history.