Let you modify the registration link of your WordPress website.
Of course not, but it is a great additional protection against them.
No! The only valid characters are [a-zA-Z0-9_-] and in general any character valid in an URL-encoded string.
Most probably because nobody submitted a translation for your language yet. But you can help. Read our guide for plugins translators at http://www.gabsoftware.com/tips/a-guide-for-wordpress-plugins-translators-gettext-poedit-locale/