Copy /theme/snap/lang/en/theme_snap.php into a new folder /theme/snap/lang/ru/theme_snap.php then translate the words that the strings equate to.
See:
- https://docs.moodle.org/dev/String_API#Adding_language_file_to_plugin
- https://docs.moodle.org/dev/Plugin_files#lang.2Fen.2Fplugintype_pluginname.php
- https://docs.moodle.org/35/en/Language_packs
or you could do this for all with AMOS: https://docs.moodle.org/dev/Translating_plugins as Snap is on the Moodle Plugin's DB and: https://docs.moodle.org/dev/Contributing_a_translation.