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.
Also Tajikistan. Ukrainian-Belarussian has much much higher overlap then Russian Serbian. And lets face it, Ukranians and Belarusians would use Russian when talking to each other (keeping in mind that Belarussian is barely used in Belarus itself).
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.