My mom is switching to Ubuntu because XP support is ending, her English isn't great but she speaks both Russian and Polish natively.
Which language is most completely translated, Russian or Polish?
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If you go to https://translations.launchpad.net/ubuntu/trusty/+translations and click "View all languages", you'll find that Russian and Polish are currently quite similar with respect to completeness in 14.04.
I'd like to also mention the language priority list feature included in Ubuntu. It's possible to choose e.g. Russian as the first language and Polish as the fallback language. If you open Language Support and click the "Help" button, you can read more about it.