Fixes mikelbring/tinyissue#118.

The error was fixed by checking that there is a user logged in before trying to get the user's language.
Fix the code indentation of the german language files.

There is no more need to make the already translated lines stand out.
Localize the activiy message displayed on an issue's detail page.

To make it possible to express that an issue was (re-)assigned "to" someone,
a new line was added to the language files for the translation of "to".