Kobo dictionaries do not support morphology - only infinitives and nominative forms are supported - making it next to useless for a highly inflected language such as Russian.
Check out the custom dictionary thread here in the Kobo sub-forum. There's a decent ru-eng dictionary based on wiktionary. It still lacks many words, but should be good enough at your level.
When you feel confident using a ru-ru dictionary, I'd recommend using Koreader and downloading the Russian wiktionary dictionary from within the app. It's the best option on Kobos.
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