Basically you need to move/copy it to your include/i18n/ folder inside your osTicket installation and change the language in "My Preferences" afterwards to Thai.But you should also note, that at the moment osTicket multilanguage support is not complete and also not officially supported yet. There is the latest stable osTicket 1.9.3. without language support and beside that there is 1.9.4-RC2 (Release Candidate 2) which support languages, but is not intended to be used in productive environments. So I suggest to be a little bit patient until 1.9.4 is released and (I guess then) multilanguage support is official.Btw. If you like to help translating osTicket you shall read this quote from Peter:The translation effort is broken into three phases to ease the process of porting osTicket to other languages.  To help with the translation and make sure Hebrew is part of the upcoming release join the project at http://i18n.osticket.com

As a reminder 1.9.4 should not be used in a production environment at this time, and translations are currently in phase 2 of 3.

@[deleted] - 1.9.4 stable will be released before phase 3 & 4 are complete. We're already dogfooding v1.9.4 in production. 

@[deleted]

Interesting, the first time I read/hear about a phase 4! Or was it just a typo?

I'm hoping that it was a typo.  Since a phase 4 would be news to me too (and contradicts every press release I've read). :)http://www.osticket.com/blog/92

@[deleted] - Phase 4 will involve adding ability to overwrite/replace some translated phrases on the fly within help desk e.g Ticket to Request. This will be useful for industry specific terminology.

Thanks for info @[deleted]