my 2¢: Russian could be assumed as a minority language for all* of ex-USSR countries, so OP missed Armenia/Georgia/Moldova.

Also, it would be interesting to see partial overlap cases (e.g. Ukrainian-Belarussian, Russian-Serbian) where one party could understand another but couldn't answer well except in "universal" sign language (or through switching to another common denominator language); though good luck with researching this one.