A couple of people submitting in French have received acknowledgement emails in English. As a kludge, I included both French and English texts for the The "New User Registration" and "Submission Acknowledgement" in the English email template by editing the appropriate items in the Prepared Emails section in the Conference Site Management.

So far we know about these language settings available to the user:
- Language [for the interface] (single select dropdown on all pages)
- Working Language in the User Registration form (checkboxes)
- Form Language in the submission form (single-select dropdown)
And these settings in Conference Site Management / Languages:
- Primary locale [currently English, other choice is Français]
- Supported locales for UI [English and French enabled]
- Supported locales for Forms [English enabled, French disabled - to
address bug in proposal submission process]

I'm not sure how the software decides which of these settings to use to figure out which language to use for each of these various emails, but there seems to be problems with the language support. Wrote detailed bug report to SC in library.