The file includes/installer/i18n/ru.json
contains "config-page-welcome": "MediaWiki проектына рәхим итегез!",
, but it is in Tatar. In Russian it should be: "config-page-welcome": "Добро пожаловать в MediaWiki!",
Topic on Project:Support desk
You can fix it yourself in https://translatewiki.net/
Thanks )