These files are used by Spring to internationalize all strings in your module. It is highly recommended you use these.
The files should be in /omod/src/main/resources.
You need to list your messages.properties file in your config.xml file in "messages" elements.
Typically they keys you define in a module's messages.properties file should start with your module's id, and any keys that do not start with your module's id will have it prepended.
Starting in OpenMRS 1.9.3 you are allowed to override this behavior, and let your module define message keys outside of its namespace. To do this, you need to include a special entry in the properties file like:
The main english file should be named messages.properties. Each other language will be messages_fr.properties (for French)
Now in your jsp files you can use <spring:message code="mymodule.doSomething"/> and it will be translated when the user chooses to use the French locale.
Most developers only know a single language. It is acceptable to only fill out the english messages.properties file and wait for another language speaker to help you out.