It seems that only donation forms can be displayed in French, as per these instructions: https://webfiles-sc1.blackbaud.com/files/support/helpfiles/rex/content/en-ca/content/donfm-languages.html

Folllowing those steps for an event registration form does not work.

Please give us the ability to have 100% French event registration forms and emails to attendees. That means all text, fields, values, links, link destination pages, buttons, etc., must be in French, otherwise we cannot use these forms in French Canada.