This error occurs when decompiled Flex Builder project is recompiled in Flex Builder because the original SWF file was compiled using the locale(s) which are not installed on a target computer.
There are two possible cases:
1) Error example:
Unable to open "C:\Program Files\Adobe\Flex Builder 3\sdks\3.2.0\frameworks\locale\fr_FR-locale"
The path displayed in the error message points to Flex SDK folder, which means that Flex compiler could not find a corresponding locale in Flex SDK on this computer, which is specified in Flex Builder project.
Solution: go to Project -> Properties, in the project properties dialog select “Flex Compiler“ and delete the arguments for the locale(s) that are not installed in your Flex SDK (i.e.: “-locale fr_FR” in this case) from those listed in “Additional compiler arguments” field.
Unable to open "C:\Users\username\Documents\Flex Builder 3\index.swf.flex\locale\en_US"
In this case, the path displayed in the error message points to the Flex Builder project folder and means that your project doesn’t contain the locale, which is used by default in Flex SDK installed on your computer; in this case it is “en_US” locale.
Solution: create a corresponding locale in your Flex Builder project.
As an option, you can use one of the locales that your project already has. For example, if your Flex Builder project features “en_GB” locale, you can rename or copy it to “en_US” and make further adjustments, if necessary.