Note: Peter announced that 1.8 will have language support.
Originally I was going to go over all the different files that you will need to translate things to fully translate your installation. However while writing the list (which I haven't finished, but may finish and post later) someone else over at the osTicket forums posted another solution. One I may add that may be very close to the solution if not the solution released in version 1.8.
User Thane posted the following thread on the osTicket forums [MOD] Multilanguage Support which has also been posted on the projects github. You should check it out as its a fantastic solution. If you have modified your 1.7ST you will have to re-mod it after you install this, but its a lot faster and easier than trying to find every 2200(ish) string that needs translating.