I can't comment on the Ukrainians because absolutely have no idea what their deal is...
Russia and China have shared a fairly close relationship over the years.
China even has a minority called: 俄罗斯族 (Russian Minority in China):
Russian ethnic minority in China
A Chinese-English Dictionary
the Russian nationality, or the Russians, distributed over the Xinjiang Uygur Autonomous Region and Heilongjiang Province
KEY, Oxford and CC-CEDICT dictionaries pretty much say the same thing(s).
Wikipedia has an article called History of Sino-Russian relations which says:
China and Russia have generally had good relations since 1991.
and links to Sino-Russian relations since 1991 which says:
Diplomatic relations between People's Republic of China and the Russian Federation dramatically improved after the dissolution of the Soviet Union and the establishment of the Russian Federation in 1991. The two countries share a long land border which was demarcated in 1991, and they signed a Treaty of Good-Neighborliness and Friendly Cooperation in 2001. On the eve of a 2013 state visit to Moscow by Chinese President Xi Jinping, Russian President Vladimir Putin remarked that the two nations were forging a special relationship.1
Geography, residency and politics aside China has also taken many Russians songs and translated the lyrics and "covered" them in Chinese. Take 三套车 for instance - if you listen to it you can distinctly tell it's Russian (it has Russia written all over it!) but it's sung in Chinese!
There's that scene in 太阳照常升起 with the Russians and the Russian song too - sorry for the ambiguity but you just gotta see it yourself.
The point anyway is that -- Russia has made quite a big impact on the Chinese and China hasn't reject them like they have so many others (English abbreviations, like NBA, or words aren't even allowed in Chinese news).
So while there are quite a lot of Russians living in China, a lot have even grown up there - you can see from there relationship that they are quite "connected". So it doesn't seem like quite a stretch for Russians to learn Chinese.