The language is detected from the browser + the user locale setting, you should read This and This, its very similar and shows you how to check in Rails, also if I look in the app\views\mailer\password_reset, RU is listed.
Hope that helps you somewhat.
from the path ‘/opt/zammad/app/views/mailer/password_reset’ which stores multi-language templates, zammad by default uses ‘en.html.erb’ file and I have moved ‘ru.html.erb’ to ‘en.html.erb’