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 join the OpenMRS team on Transifex. (If you have trouble doing this, please ask for help on OpenMRS Talk.)
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.
Want to do more?
We're looking for an I18N Manager, to help coordinate the different translation activities. This is a volunteer position that can probably be done in a couple hours a week. See this post on OpenMRS Talk for more details.
Do you want to be a Coordinator or a Reviewer for one language in Transifex? Once you've been around the OpenMRS community for a while, and done some translations, we're happy to promote you. Just ask on OpenMRS Talk.
For more details about how a developer would configure a module to use Transifex, see Maintaining OpenMRS Module Translations via Transifex.