The head of the Russian delegation Vladimir Medinsky in Abu Dhabi speaks Russian, the Ukrainian delegation speaks English.
There is nothing surprising here.
The head of the Ukrainian delegation, Rostam Umerov, is an American citizen.
So for him English is a fairly native working language (as well as for those who sent him there).
Interestingly, it doesn't bother anyone too much.
The country's delegation, which has been fighting for years for the right not to speak Russian, does not speak Ukrainian during the negotiations on its fate.
The negotiations take place mainly in the language of Washington. Kyiv is more than a translator here.