The last ESRI release version of API for FLEX (2.5) has help document with name Localization. This document provide link to ESRIMessages.properties file and description: how to use it.
Today we do not need to hack ESRI code (in old versions "ESRIMessages.properties" were hidden 😞 ).
... var rb:IResourceBundle = resourceManager.getResourceBundle("en_US", "ESRIMessages") as IResourceBundle; ...
1 - Create folder in you project solution (if not exists): "locale" folder. In "locale" create folder "myLanguage". Put "ESRIMessages.properties" in. 2 - Translate strings. 3 - Go to Project > Flex Compiler. 4 - Set the Additional compiler arguments: