Setting the argument '-nl en_US' should retrieve all the messages and strings in English, irrespective of the OS language. This param can be set either in FlashBuilder.ini file (that is available along with executable), or either can be pass through command line.
I would recommend to set this parameter in the .ini file as it would persist the parameter across restarting sessions.
If you are on Windows, FlashBuilder.ini file is available in the <installation location>.
On Mac, to find the .ini file -> right-click on "Adobe Flash Builder 4.5" executable and select Show Package Contents. Then traverse to Contents->MacOS->Adobe Flash Builder 4.5.ini
Most likely, the FlashBuilder.ini file should already have -nl argument. So just changing its value to 'en_US' should suffice. Hope this helps. Let me know if that does not solve your usecase.
Unfortunately, this did not work for me. Here are the contents of the FlashBuilder.ini file:
Whe you say:
This param can be set either in FlashBuilder.ini file (that is available along with executable), or either can be pass through command line.
do you mean set it as a compiler directive/option?