Simple formula question / Template conditional statements

Simple formula question / Template conditional statements

I want to achieve to return something else than "true" or "false" in my email templates sent to clients. I have a check-box in my module that indicates if an activity is completed or not.

I'm checking if a formula can be my solutions, but are struggling to create the right syntax. In English the syntax would be something like:  If checkbox ${CustomModule1.Client Payment 1} is true, then return "Payment received", if not, return "Payment not received yet".  How will this formula syntax be?

What I really want to achieve is to use this conditional statements directly in my templates, to achieve the different templates per language I have.

Can you please assist me in finding the best method to solve my issue?