There would be a few manual issues to get RText running in Eclipse. I've not worried about this too much myself, because I tried to use RText itself while developing it (more usage == find more bugs
I believe you'll need to add both RText/i18n and Common/i18n as Source Folders in Eclipse (without the "org" folder one level deeper). Your next problem would be that, at runtime, RText assumes some resources can be found at relative paths to the directory it's running from (XML files, for example). And I think the source distribution has these resources thrown about in various (different from their runtime) locations.
I can see about making the source distribution more Eclipse-friendly, I think I'll have some time to do so in the next couple of days. I can help you figger it out if you need some help, and maybe make the next "official" release more Eclipse friendly as well (should be very soon).
Another idea is to use Eclipse to build the RText distribution via Ant (Alt+X, Q from RText's build.xml in an editor), then create an External Tool that runs RText/dist/RText.jar. But unfortunately that won't let you interactively debug the code.
As to why it's looking for _en.properties files when there are none, it seems to just be the way the error message in the stack trace from ResourceBundle is designed. It always says "en" (probably really the system default Locale?) even though it always looks for default properties files as well before throwing the Exception.