System users can simply select their language by using the flag in the system. This affects the system, and also all e-mails they receive from the system.
This is not the case for people who are registered in the Competency module, who do not have ABMS users.
However, setting a language for these users is very simple.
On the user profile page, top section, bottom left, there is a toggle to decide whether or not this person should receive e-mails at all, and a drop-down menu in which you can select their language.
Simply make your choices and click save in the bottom left corner of the top section (personal information section).