My understanding is that this actually could only be controlled via a special department.
You create a department for a different language and create a different email template („Add New Template Set) with the wanted language for this department. In the department you can assign this template set.
Use a separate email address for this department for users with this other language. The same with the canned responses: create separate canned responses for the other language and make it available for this department.
So all tickets which are routed to this department will automatically use the correct language for the email templates and the corresponding canned responses are available as well.As far as I know the multilanguage feature is not done.The last thing I heard from this was that it is in phase 3 (of 4): http://forum.osticket.com/d/discussion//language-official-translation-phar-is-bad-how-to-use-custom-translation
Maybe then there is another way but I don’t know because I could’t find informations about that..