Felice,
The COM_JFBCONNECT_LOGIN_USING_FACEBOOK string is defined in the com_jfbconnect.ini file. It's there simply because, instead of the SCLogin module, you can simply use {JFBCLogin} to add the Login With Facebook button. Since that can be anywhere, we needed a more global language file.
As for the MOD_SCLOGIN_WELCOME, I'm not sure why that's not translatiing. If you would, try copying and pasting the exact line below:
MOD_SCLOGIN_WELCOME="Welcome %s,"
That's the original line from our translation, which should work. If it doesn't, check the following:
* There are no errors in your ini file. If there are, Joomla won't be able to parse all strings, and that could be why that one doesn't work
* Don't use a double quote " mark in the string, or really, any special characters. If absolutely necessary, at least remove them temporarily to see if that works
* Make sure there's no space around the = or at the end of the string
* Make sure there is a %s in there somewhere (which will be replaced by the user's name).
Hope one of those work for you, but if not, let us know!
Thanks,
Alex