What ethnic lines? Some people speak Russian at home, but still think of themselves as Ukrainians. Their neighbors may speak Ukrainian or Surzhyk, a Russian-Ukrainian mix. Some areas speak Russian, but are surrounded by Ukrainian-speaking areas. Parents and their children may not have the same preferred language. Russian-speaking Crimea shouldn't have been given to Ukraine by Khrushchev, but in the rest of the country language areas may change over time. "Ethnic" identity is even harder to determine and harder to make the basis of boundaries.
St Z is a Russian speaker, he had to learn the Ukrainian language late in life…much said about his piss poor Ukrainian….the irony, a Ukrainian President who only spoke Russian the majority of his life!