Flash Builder does not provide any setting to select the langage of your choice. The used locale is based on your OS locale. To select the locale of your choice (either to get the message in your own locale, or to get the default english error message, which are often more useful), you can just add a locale argument to your Flash Builder shortcut/alias :
Your default Flash Builder shortcut should be somethinf like this :
"C:\Program Files\Adobe\Adobe Flash Builder 4.7 (64 Bit)\FlashBuilder.exe"
You only have to add this argument :