Anyone writing a custom jmeter component has to make resource strings. With TestBean, it's easy enough, but for regular gui components, their only current option is modding the messages.properties file and recompiling JMeter. External developers should have the option of specifying a custom resource file/location that jmeter tries to get properties from first, and then fall back on the given jmeter resources. This would also allow anyone to override the messages.
Gui components can override "getStaticLabel()" to retrieve the message from a private resources file.
*** This bug has been marked as a duplicate of bug 52161 ***
This is not the same as Bug 52161, which is about upgrade.properties, not message.properties
Not really needed knowing all plugins that currently exist.
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/1390