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.
I accidentally looked at the Dative line in the dictionary for редкое and явление - You're right. Though I don't understand why you claim it's nominative where there is clear negation here. Negation is genitive.

"люди из советских корней" - the meaning of what you said is as if the people were made of Soviet plants' roots.
And I completely stand by it. They are really hardy people! Ever tried to arm wrestle one? I swear it's like they're made of roots! Can't bend them...

Fine. I made a mistake here. Will use "с" next time.