How was the language of my existing templates set after the update?

The system set the language of each existing template automatically, once, when the Template Language setting was added, based on the text written in it:

Template content	Template language set
Contains Arabic characters	Arabic
Contains Latin characters only	English
Contains no text, or its language is unclear	The system language of your account

To change the set language, open the template from Templates → Printable Templates for editing, select the language from Template Language, then click Save.