Earlier I posted how to do Flash localization. Now it's time for an example of Flex localization. Flex localization is a bit more difficult to do, but it's a lot better done. Follow along with the example attached at the bottom of the post.

This example is for Runtime localization. This means that the resource bundles will be external swf files and will not be compiled into your main application.

The official flex documentation for localization can be found here.
I plan on just writing a quick-start example.

Localization file structure

Create the folders and property files you'll need. In this example, let's use English and German.

yourFlexProject
  bin-debug 
  src
  libs
  locale
      en_US
         myBundle.properties
      de_DE
         myBundle.properties

Creating the property files

Create the .properties files in their respective folders, and then set their text file encoding to UTF-8. If you don't do that, characters with accents and such will just show up as empty squares.
Right click the file, then go to properties.

# locale/en_US/myBundle.properties
greeting=Hello
goodbye=Goodbye, come back soon!

# locale/de_DE/myBundle.properties
greeting=Hallo
goodbye=Auf wiedersehen!

Compiling the resource bundles

Now here's the tricky part. Flex Builder has no easy way to compile these resources we just created automatically. So the best way to do this is using ANT.
ANT will come installed by default in Eclipse, but in Flex Builder, you will need to add it. Flex Builder comes with some strange crippled version of ANT, I don't quite understand that.
Installing Ant in Flex Builder Stand-alone

Once you have ANT installed, you will need to create your script to tell the Flex compiler to create swf files out of your language bundles.
In order for ANT to know how to use the flex compiler, we need to copy two files over from your flex directory.
In your Flex Builder 3\extras directory, there are two files; flexTasks.jar and flexTasks.tasks. Copy flexTasks.jar to your flex project libs directory (the directory should exist by default), and copy flexTasks.tasks to the root directory of your project.

Now that ant is installed and has what it needs, create an xml file in your root project directory called antBuild.xml:

<?xml version="1.0" encoding="utf-8"?>
<project name="Example resource bundle builder" basedir="." default="main">
	<taskdef resource="flexTasks.tasks"
		classpath="${basedir}/libs/flexTasks.jar"/>
 
	<!-- CHANGE TO YOUR FLEX DIRECTORY //-->
	<property name="FLEX_HOME" value="C:/Program Files/Adobe/Flex Builder 3/sdks/3.0.0"/>
 
	<property name="APP_ROOT" value="myApp"/>
	<target name="main">
		<antcall target="en_US"></antcall>
		<antcall target="de_DE"></antcall>
	</target>
	<target name="en_US">
		<mxmlc>
			<locale>en_US</locale>
			<source-path>locale/{locale}</source-path>
			<include-resource-bundles>mainBundle</include-resource-bundles>
			<!--<include-resource-bundles>anotherBundle</include-resource-bundles>-->
			<output>bin-debug/Resources_en_US.swf</output>
		</mxmlc>
	</target>
	<target name="de_DE">
		<mxmlc>
			<locale>de_DE</locale>
			<source-path>locale/{locale}</source-path>
			<compiler.library-path dir="${FLEX_HOME}/frameworks" append="false">
				<include name="libs" />
			</compiler.library-path>
 
			<include-resource-bundles>mainBundle</include-resource-bundles>
			<!--<include-resource-bundles>anotherBundle</include-resource-bundles>-->
			<output>bin-debug/Resources_de_DE.swf</output>
		</mxmlc>
	</target>
</project>

To use this script, open your ANT view, click the Add Buildfiles button, and add that xml file we just created. Because we set default="main", we can just double click the "Resource bundle builder" icon we now should see in the ANT pane.
If everything worked, there will be two new files in your bin-debug directory: Resources_en_US.swf, and Resources_de_DE.swf

If you've gotten this far, the hardest part is over, so keep with me here =-)

We now need to tell Flex where to look for the bundles. For that, we need to add a FlashVar entry.
I know this is annoying, but there's three places we need to modify to change FlashVars in Flex.
Open your index.template.html page. Here you will see three major parts:
Version Check
if ( hasProductInstall && !hasRequestedVersion )
AC_FL_RunContent
Javascript Embed
else if (hasRequestedVersion)
AC_FL_RunContent
And noscript HTML object

The flashVar parameters we need to add are: resourceModuleURLs=Resources_en_US.swf&localeChain=en_US
This will tell Flex what locale to load first on startup.
If you know how to add that into the three places, go ahead, if not, look at the index.template.html file in the example I have attached at the bottom.

Now we need to tell Flex not to compile any language files into the main application. To do this, go into your project properties, flex compiler, and under additional compiler arguments, change -locale=en_US to simply -locale=

Using the resource strings

Ok, the localization should finally be ready to use!
For mxml components, use binding expressions to bind the localized strings:

For actionscript, just use:
resourceManager.getString('myBundle', 'greeting')

If the locale changes at any point, the text will automatically change, at least that part is easy.

To change the locale at runtime:

var eventDispatcher:IEventDispatcher = resourceManager.loadResourceModule("Resources_de_DE.swf");
var localeChain:Array = ["de_DE", "en_US"];
resourceManager.localeChain = localeChain;
eventDispatcher.addEventListener(Event.COMPLETE, languageChangeComplete);

So there you go. Setting this up for the first time can be a troublesome process, but once you know how, it's very easy to use.