You App should not worry about Email, SMS or Telegram. Just let Filipid handle it.
Deal with a single type of templating system for all channels, and all languages
You can also set the default channel preferences for each notification.
For each notification, the user can choose to enable or disable any given channel.
And if they don't want to bother about how to receive the notification, the defaults for each notification will be used
Each user has also his own set of data, that allows to select int which language to receive the information, and the info required for the different channels (email address, phone number...)
Along that data, you any other fields can be added that will be merged with the notification payload before rendering the templates.