3
Yes, user.php, but there is also quite a few different error messages related to actions in the other language files.
I.E. "You don't have permission to access this area" is in:
language/english/global.php around line 58
Some modules have their own defines in their language folder.
I would recommend getting crimson editor, you can do a "find in files search" and lots more. If you search for Crimson Editor in google you should find it.