Hi,
try to configure language of the email template (Ticket: Reception Acknowledgment?). It should be:
${object.sudo().partner_id.lang}
Take into account that all other recipients (if they exists) would also receive this message in a partner language.
UPDATE
To construct complex structure, you should learn a little Mako: https://docs.makotemplates.org/en/latest/syntax.html
In your case it should be something like (not tested):
% if object.sudo().partner_id.lang ${object.sudo().partner_id.lang} % endif % if not object.sudo().partner_id.lang fr_CA % endif
Or use Python inside like:
${object.sudo().partner_id.lang and object.sudo().partner_id.lang or "fr_CA"}