All language string should be localized and stored in the language section. You have a full featured language editor with every installation of OpenMeetings, check out the LanguageEditor .
To lookup the labelids in the GUI you can simply run OpenMeetings client with the debug enabled. That way every text-string has the labelid in brackets additionally in the textfield. To start the client in debug mode please check "Tips and Tricks" section of Installation guide
If you would like to change the standard email layout or add for example some footer in every email you have to change the email templates.
To do that: