Hi
I found the Addon Locale2MUI that allows what we all need... But this need to be packaged into a MSI file for deployment.
Are you able to create a single MSI with Firefox (English) + Locale2MUI + All Language packs?
The latest version of Locale2MUI is not on Mozilla.org, but the developer of this module has an updated version that works with Firefox 8.x, too.
Regards
Marc