The OTRS framework allows for different languages to be used in the frontend. The translations are contributed and maintained mainly by OTRS users, so your help is needed.
Starting with OTRS 4, all translations of the OTRS GUI and the public extension modules are managed via Transifex. The OTRS project on Transifex can be found at https://www.transifex.com/otrs/OTRS/.
To contribute to a translation of the OTRS GUI, an extension module or a manual, please sign up for a free translators account on Transifex. Then you can join your language team and start updating your translation. No additional software or files required. The OTRS developers will download the translations from time to time into the OTRS source code repositories, you don't have to submit them anywhere.
If you want to translate the OTRS framework into a new language, you can propose a new language translation on the Transifex OTRS project page. After it is approved, you can just start translating.