We have a static class:
com.earth2me.essentials.I18n._

This is called by:

_("broadcast", <text>, <sender name>));

This pulls the "broadcast" translation key from the correct file, and replaces the variables.

You can find the broadcast implementation here:

https://github.com/essentials/Essentials/blob/2.x/Essentials/src/com/earth2me/essentials/Essentials.java#L712