I could find the constant for translating/changing the connect button text in your language files(MOD_SCLOGIN_LOGIN) but I could not find it for changing the text of the log out button.
JLOGOUT is actually a language key in the Joomla core files. It's not included in mod_sclogin language folder. It should be found in the /language/xx-YY/xx-YY.ini file.