  The use of language is innately political and often a vehicle of cultural
identity as well as the basis for nation building. Here, we examine language
choice and tweeting activity of Ukrainian citizens based on more than 4 million
geo-tagged tweets from over 62,000 users before and during the
Russian-Ukrainian War, from January 2020 to October 2022. Using statistical
models, we disentangle sample effects, arising from the in- and outflux of
users on Twitter, from behavioural effects, arising from behavioural changes of
the users. We observe a steady shift from the Russian language towards the
Ukrainian language already before the war, which drastically speeds up with its
outbreak. We attribute these shifts in large part to users' behavioural
changes. Notably, we find that more than half of the Russian-tweeting users
shift towards Ukrainian as a result of the war.
