You can change the overall language that Facebook uses, but you can't completely customize the text in those buttons. JFBConnect will automatically try to set the language of the buttons to that of the user's browser language or use your Joomla default language setting.
If you want to override the language detection, you can force the Facebook language value in the Configuration -> Facebook area of JFBConnect.
I hope that helps answer your question, but should you need anything else, just let me know.
Thanks,
Alex