Automatic language expression

[Node-BonBon]

Hello,
I understand that the language strings in nodebb are automatic, meaning that one has to write an expression and enclose it in [[]] or {{}} (not entirely clear to me), and the forum shows the user the sentence according to his language definitions. I need to print in the app.alert message the string that appears when receiving an email from the NodeBB forum - "You have a new alert" appears on the subject. What is the sentence that needs to be written to print this message and it will be displayed according to the user's language choice?