My understanding is that you have to use a own departement for each language and setup for each departement a different email address or alias. Then you clone the email template set for each departement and change only the content in the email template sets with the translated text you want. After that assign each email address and template set that you have created for the appropriate departement and then the language which is used in the notification mails is depending on the departement that the ticket belongs to. You see no modification is needed to accomplish this.
By the way: your osTicket Version is outdatet. 1.9.16 is actual for this branch and when 1.11 will be released the 1.9.x branch is not longer supported.