I recently made a suggestion to support.dotnetnuke.com that Language Editor have a Search function. Then it could be easier to locate language that we Admins see on screen messages, or email notifications - language that we see the need to modify.
Other keys have been easier to find, but this one really escaped me for the longest time. Here it is - the key to changing the 'User Name' language on the default Register page (and other Register screen language text, too):
\admin\Users\App_LocalResources\User.ascx.resx
UserInfo_Username.Text
Now, if I could just find where we can modify the Help/Tooltip for the Register screen 'User Name' field...
UserInfo_Username.Help
Mouse-over tooltip still says 'Username' and I'm not sure where to change that one, but I'll keep looking...