well, there is no default option to switch to russian on ukrainian smartphone, will switch to english.
but you may manipulate Locale
(so language), check out HERE or HERE
you can also introduce ukrainian translation "oficially", by creating strings.xml
in proper folder and just copying whole russian translation into that. but then you will be maintaining two identical, but separated translations, maybe you can use some gradle script for copying russian translation file into ukrainian translation/resources folder during app building...
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