Considering course/mod.php (line:178) and course/modedit.php (line:281),
it would better to use "module" instead of "instance".
$string['cannotupdatecoursemodule'] = 'Could not update the course module with the correct section';
$string['cannotaddcmtosection'] = 'Could not add the new course module to that section';
$string['cannotaddrss'] = 'You do not have permission to add rss feeds';
-$string['cannotaddnewmodule'] = 'Could not add a new instance of $a';
+$string['cannotaddnewmodule'] = 'Could not add a new module of $a';
$string['cannotaddnewinstance'] = 'Could not add a new instance of $a';
$string['cannotsaveconfig'] = 'Problem saving config \"$a[0]\" as \"$a[1]\" for plugin \"$a[2]\"';
$string['cannotsavecomment'] = 'Cannot save comment';