Russian was the international language during the Soviet era. It was an official policy of the Soviet government which they called ‘merger of nations’. Many years back, there was a time when there were only two Ukrainian schools in whole million-habitant city.
And almost half of Ukraine’s population uses Russian as a first language. For many of Ukrainians, Russian is their basic everyday language, or it was before the war.
With that said, you will, probably, not find anybody in Ukraine who doesn’t understand Russian – understanding it is different from perfectly speaking it.
Ukrainians can be considered bilinguals speaking Ukrainian/Russian. They understand Russian because their parents were mostly born in Soviet Union, and even the modern Ukrainians most speak a Russian-Ukrainian creole – many of them speak Russian when they are paying attention, but slip into creole when excited or upset.
Ukrainian is dialectically different from Russian, there is a large degree of mutual intelligibility between them. In written, the mutual intelligiblity of the two languages is 60–70%, depending on topic and the educatedness of the speakers.
Since in the Soviet times, Russian was the official language in Ukraine, older Ukrainian citizens speak and understand Russian very well. Most of Russians, mainly young soldiers from far-Eastern territorries of Russia, however, do not speak Ukrainian, of course.
Since Ukrainians, partly due to the political past, understand spoken Russian much better than Russians understand spoken Ukrainian, it is much more likely that the language of communication is rather Russian between invading soldiers and civilians.
Nevertheless, if a Ukrainian person who does not speak Russian at all speaks Ukrainian slowly and clearly, Russians tend to understand at least 30–40% of his or her speech.
Nevertheless, it is still a tendency that a Ukrainian native speaker with zero learnt Russian knowledge understands spoken Russian better than a Russian native speaker who has never learnt Ukrainian understands Ukrainian.
So in most cases they probably communicate in Russian. As mentioned above, in the case of written texts, they understand 50–60–70% of each other’s language, similar to the degree of mutual intelligibility of written Spanish, Portugues and Catalan.
