If you build your Flex application using flexmojos version < 3.7 you have to give Flashbuilder some additional compiler settings for multilingual application.

In my projects I use the standard flexmojos maven project structure. I put my locale properties files in
src/main/resources/locale
which means you have for german
src/main/resources/locale/de_DE
and for english
src/main/resources/locale/de_DE.

For this setup you have to give Flashbuilder the additional compiler arguments
-locale=en_US,de_DE -source-path=..\resources\locale{locale} -allow-source-path-overlap=true.

If you use GraniteDS the granite-essentials.swc has to be compiled in your application. In my project setups I put the library in
libs.

The additional compiler settings including the settings for the locales and the GraniteDS library is this
-locale=en_US,de_DE -source-path=..\resources\locale{locale} -allow-source-path-overlap=true -include-libraries ......\libs\granite-essentials.swc