Russian is also an official language in Belarus, so you should count them in.
Ukraine is a different story.
You're wrong:
If we're counting this by language, we should figure out how much Ukrainians (or Belorusians) use internet in russian language, primary, and how much don't; and then we should sum up those numbers. Everything else would be a speculation.
So we then we should count ukrainians and belorussians separately.