Russian, no contest. Not quite where you mention (presumably thankfully!) but not far off:
The Russian language in Ukraine is the most common first language in the Donbass and Crimea regions, and the predominant language in large cities in the East and South of the country.
In Ukraine the ratio may be nearer 50/50 but there are approximately five times as many Russian speakers as Ukrainian, so could be more likely to be useful later.
From here, p85:
While there is greater freedom in public encounters, in many contexts there
is pressure to adhere to a particular language. In Kyiv, the country's capital,
Ukrainian is the language of written official communications, public political
communications, and formal interactions in institutions such as schools,
courts, and the media; meanwhile Russian is the predominant language of
informal written and spoken exchanges, and of economic power (Zazulya,
2003: 138). While the language of the public political sphere is increasingly
Ukrainian, the language of business remains predominantly Russian.
Ukrainian functions as the language of instruction used during lessons and
in official meetings in most Ukrainian schools. However, in most urban
schools in central and eastern Ukraine, Russian is the dominant language
outside of formal classroom interactions. Schoolchildren generally shift to
Russian during breaks and unofficial events. In many cases, principals and
teachers do not encourage the pupils to use Ukrainian, try not to pay attention
to the language shift, and respond to pupils in Russian as well. Ukrainophone
parents in Kyiv have reported that their children feel uncomfortable speaking
Ukrainian in school for fear of being ostracized (Bilaniuk, 2005: 4748, 63-64).
Children who withstand the pressures of linguistic conformism and use
Ukrainian in such russophone language environments are rare. In this context,
according to Masenko (2004), Russian can be defined as a language of
accommodation (prystosuvannia) while Ukrainian is a language of opposition
(protystoyannia). Meanwhile, the prevalence of Russian in informal contexts
does not allay the concerns of those russophone parents whose children are
taught in Ukrainian and receive little or no formal education in Russian, and
as a result, cannot write correctly in Russian.
That research is a little dated but since much is about schooling may relate to those who are now adults.
Being able to convert Cyrillic script into something more familiar could be very useful but that applies to both languages. Календарь may look “Greek” but mere character substitution (to kalendar) would give a good clue to its meaning. The alphabet should be easy to learn in the time available.