Valda, you do not need genitive here, only nominative. And Genitive of "редкое явление" is "редкого явления". "Редкому" is Dative.
So you put "двуязычие" in Nominative, "редкое" in Dative and "явление" in Genitive, while all the three words should be in one case, Nominative.

"люди из советских корней" - the meaning of what you said is as if the people were made of Soviet plants' roots.

The idiom actually means the the people HAVE Soviet roots as a tree can have roots (which still bound them to their birthplace), not that they are MADE of Soviet roots.

Even in English you say "people WITH Soviet roots", not "people MADE FROM Soviet roots".