Complex sentence: Europe is also where a multiplicity of cultures, nationalities and ethnic groups originated outside of Europe reside in, most of them are recently arrived immigrants in the 20th century and their country of origin are often a former colony of the British, French and Spanish empires.
Split into simpler sentences: Europe is also where a multiplicity of cultures, nationalities, and ethnic groups not indigenous to Europe reside. The majority of them are recently arrived immigrants in the 20th century, usually from former colonies of the British, French, and Spanish empires.