When sending notifications, how to determine in which language to whom to send email?

Question

Email

There is a service. The service has both Russian-speaking and English-speaking users registered. The service is localized for both those and those. It is necessary to make a mass mailing about the service update to all registered users. How to determine to whom and in what language to send a notification?

Answer 1

If the interface is localized, then it is most logical to send a notification in the language in which the user works with the service, so as not to cause language dissonance in him.
The registration language may not be suitable in a situation where the user, for some reason, registered in a language other than the language in which he uses the service (for example, during registration the service was not localized, the user helped with registration in another language, the user could not find the language switcher during registration, etc.)