ideally stick to within your broader tribe (germanic with germanic, slavic with slavic, celtic with celtic etc), but nah european with european has never been a problem. A german marrying an englishman or a dutchman has pretty much no effect on the gene pool. They can assimilate totally.
Though I'm going to side eye couples like pewdiepie and marzia (swede with dark italian)
Someone pull up that map of genetic distances within europe.
side note: celtic is already a multi-ethnic tribe.
I think they are a mix of Beaker-bell people and Iberians who got heavily culturally enriched by vikings, but I may be wrong about that.
That's fine, so ideally isolate that within the celtic group.
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