The only time the Ukrainian would have to pick the Russian flag would surely be if they selected the Russian language, and not the Ukrainian language which is not the same as Russian and doesn't even use the same alphabet. The languages and alphabets are undeniably similar and somewhat mutually intelligible, but they are not the same.
Only time this is a problem is when the Ukrainian localization isn't even supported and the Ukrainians are forced to use Russian, but then isn't that the bigger problem?
I know Ukrainian and Russian are different languages, that’s why I specified “a Russian-speaking Ukrainian”. Plenty of Ukrainians are native Russian speakers.
While the Russian-speaking Ukrainians would indeed speak "like the Russians", I'd at least go off the limb and say that they'd probably prefer to have their native language in the UI represented as something like "Русский" instead of the Russian flag. Like if we were to ask a randomly picked Russian speaking Ukrainian like, say, Volodymyr Zelenskyy about how he'd feel about his native language being represented by the Russian flag, I could imagine him being sort of annoyed by that even if he'd probably understand the reason behind the choice.
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I feel that like many other people I'm this thread, it'd probably be better to have the language be represented as itself in the "Select language"-menu instead of relying on flags. It would be more accessible, allow for choosing different dialects of the same language, avoid faux pas like the aforementioned thing with Russian-speaking Ukrainiand, and so on. Of course you'd also want a translated name of the language in the UI which would correspond to the current UI language, but having the language there in its native form is a must.
Only time this is a problem is when the Ukrainian localization isn't even supported and the Ukrainians are forced to use Russian, but then isn't that the bigger problem?