Gerard,
We've tried to do a bit of testing, but haven't been able to recreate your problem. If the English language pack has been uninstalled from your site, the default language (whatever that is) shouldn't be used.
JFBConnect does work with the System - Language Filter plugin (part of Joomla), which will attempt to detect the language of the users browser. If detected *and* that language exists on your site, JFBConnect will set the new user's language to their browser language. It's meant to make things easier on the user by using their language by default. There is nothing hard-coded in JFBConnect to always use English at all.
If that doesn't seem to be the case for you, or if English isn't even available on your site, then I'd be unsure of the answer. Would it be possible to get admin access to your site to understand your configuration better?
Thanks,
Alex