This is nonsense... Elvish language, such as Huttese, Jawaese, Ewokese or Mandalorian are unreal languages, they don't exist in our world even if they had been created based on existing tongues. No invented language has been voice-acted in French or something, nor in LOTR neither in SW : Jabba speaks in Huttese, Elves in Elvish. We translate and dub the English language only. For some, we have subtitles to understand what they are saying but that's all.
So, again, there is no problem for DP to be in Russian, because all voices come frome human or near-human species which are speakin "Basic". And basic is just the word to say "human language, such as English and others". Moreover, as Numberwas explained, it is a hard task to find a cast in your own mother tongue, so in another one it becomes impossible. The project is more than 10 yo, I'm not sure the team is enjoyed by taking more years to find English speakers and waiting for their records, making the post-production etc.