The strings "COM_JFBCONNECT_THANKS_FOR_SIGNING_IN" and "COM_JFBCONNECT_EXISTING_USER_INSTRUCTIONS" should be set in the language files installed with JFBConnect. I'm assuming we don't have a translation for your language. However, the English strings in /languages/en_GB/en_gb.com_jfbconnect.ini should be used instead. I'm not sure why they aren't.
What I'd recommend is copying the language file above to your language's directory and edit the file there with whatever translations you'd like to use. After you've made the updates, feel free to share the changes back with us, and we'll make sure they make it in the next release of JFBConnect.
I hope that helps explain,
Alex
Loggin with fb
