Wiki plugin: fixed missing translatable string and danish language file

I've completed a Danish translation (da_DK) of the wiki plugin and came across an untranslated string on line 1344 in wiki-include.php and changed it to:

$content .= '<input type="submit" class="button-secondary" value="'.esc_attr(__('Compare Revisions', $this->translation_domain )).'"/>';

Let me know if you would like to include the translation in a future release.