My region is Ukraine so it is not a "mistake" that there are some relevant search results in Russian. However, can I filter out any results in Russian language anyway?
The easiest way is to add lr=-lang_ru
parameter to the search url. E.g. in Google Chrome:
Go to settings->search engine->manage search engines.
Click "add".
Put any name you want.
Put any keyword you want.
Use this for "url":
{google:baseURL}search?lr=-lang_ru&q=%s&{google:RLZ}{google:originalQueryForSuggestion}{google:assistedQueryStats}{google:searchFieldtrialParameter}{google:iOSSearchLanguage}{google:searchClient}{google:sourceId}{google:contextualSearchVersion}ie={inputEncoding}
Make your new search engine default.
Replace "lang_ru" with any language you like to exclude.
Should work pretty much the same way for other browsers. Something like
https://google.com/?lr=-lang_ru
or
https://google.com/?other_parameter=other_value&lr=-lang_ru
should work fine most of the time.