Just drop the "RB" from the class name in the example Patrick gave you. *RB.rbInfo files compile into *.class files.
String resourceBundle = wt.part.QuantityUnit.class.getName();
String key = "ea"; // or "l" or "kg" or "as_needed", etc.
String localizedName = wt.util.WTMessage.getLocalizedMessage(resourceBundle, key);