I would like to make a complete backup of my forum in case of problems.
What should I save?

I imagine well on the database.
The language file and theme.

But then I don't know if there is anything else to do.

Thank you in advance for your answers.

If you are unsure how to generate a full backup of your database and files via your server. Please contact your hosting provider for assistance. 

I just learned that my host does one every day and that it is accessible via CPanel and the JetBackup tool.

THANKS.