In the past, contributing to OTRS translations was not very easy. One needed to know where to find the correct files (in the correct branch/version) to translate for OTRS or the various modules. The files needed to be sent to a mailing list and manually included into the source archives by OTRS developers.
Since OTRS 4, translation management was migrated to transifex, a great platform for online translation management. It is free for open source projects and translators.
Since the migration to transifex 77 translators have joined and have been busy updating the existing translations and even adding a new one for Swahili, which will be released soon.
Another very nice benefit is that all the different resources, even including the manuals, can now be viewed and translated by the different language teams in one central place.
Translation itself happens in a very nice and powerful screen with filtering capabilites, suggestions and other nice features. From time to time, translations will be synchronized into the source repositories and published.
If you want to join the translation efforts, you can sign up for free at transifex and request to join a language team. Please note that from OTRS 4 on, translations will be managed exclusively via transifex.