The modules that make up the Reference Application are (mostly) translated via the online tool Transifex. It lives here:
https://www.transifex.com/projects/p/OpenMRS/
If you would like to help translate, you need to create a (free) account on transifex and ask (on OpenMRS Talk) to join the OpenMRS team on Transifex.
If you find a Reference Application module that is not included in the Transifex project, please ask on OpenMRS Talk to have it added.
The translations that you make on Transifex need to be manually pulled into modules by a developer, so after you have reached a good stopping point with a module's translations, you may want to notify its owner, and ask them to pull them in.
For more details about how a developer would configure a module to use Transifex, seeMaintaining OpenMRS Module Translations via Transifex.