Hi Yuliya,
It seems you have discovered two bugs in the Upcoming events block - well done!
Regarding the untranslated string lessoneventopens, the best solution would be for you to contribute the translation to the Russian language pack here: https://lang.moodle.org/local/amos/view.php?t=1597846365&v=l&l=ru&c=lesson&s&d=lessoneventopens You'll need to create an account on the Translation site if you don't have one already.
(As mentioned in the Language FAQ, most likely your language customisation didn't work because of caching on the server. Sometimes purging caches in Moodle isn't enough.)
As for the string containing 'closes', I tried experimenting with the Upcoming events block and switching languages and found that the words often didn't display in the correct language. I found an old report of a similar problem, MDL-18589, which was closed due to inactivity. Thus, I'd encourage you to create a new issue so that developers can investigate and fix it.