You are making your case on an example that doesn't exist, and we need to define your terms as "left" and "liberal" are no longer interchangeable terms. If by liberal you mean democratic socialist, then no those are not successful even when ethnically homogeneous EG Sweden and Denmark abandoned socialism decades ago as their economies started going the way of Venezuela.
An exception to the rule might be Norway but their system of government is irrelevant to their citizenries standards of living as they have vast oil wealth and a small population, they are basically the Kuwait of Europe.
If we want to look towards actual ethnically homogeneous countries we can't look towards either the EU or the US we have to look towards Asia EG Singapore, Japan, Taiwan, and South Korea, I am not entirely sure the economic policies of South Korea and Singapore but I know that both Japan and Taiwan have highly nationalistic right wing governments, and I'm pretty certain Singapore does as well as they have one of if not the most free markets in the world, South Korea I think has a left leaning government but to my recollection they are currently experiencing economic stagnation.
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