Well, it does not work as you describe. We have separate language template sets (DE, EN, FR) and configured per department (DE, EN, FR departments).
Our page: https://support.pambill.com/?lang=en
As you see, we add the lang variable. But still, when a new ticket is created on the https://support.pambill.com/?lang=en page, the ticket confirmation email sent is always in the default language set (German in our case) and not in English.