change "or" in register.tpl.php and in login.tpl.php into a translatable string

[JorisHeijne]

login.tpl.php: line 16
register.tpl.php: line 4

And there is already an "or" present in the .po file:

#: templates/admin/form.tpl.php:103 templates/admin/form.tpl.php:168
msgid "or"
msgstr ""