fix english translation keys

the translation key in the mail view is generated with

````ruby
link_to(t(:"text_#{@content_type}_for_meeting",
   ...
````

@content_type is a string, for example: "meeting_agenda"
The translation keys must be named accordingly
fix wrong pluralization in german translation
Merge pull request #57 from finnlabs/fix/15953_wiki_mail_translations

Fix/15953 wiki mail translations