Extension of the admin interface to maintain I18N values

Please do not yet merge this, the task is mostly completed, but there are still some minor issues:

* [x] Return default language values, if a value is not available in the requested language
* [x] ~~The build is currently failing in OpenJDK 6 due to a [MissingResourceException](https://travis-ci.org/autermann/SOS/jobs/28308622#L4710)~~
* [x] There are still some issues regarding ISO 3166-1 alpha-3 handling
* [x] `+` button does not work in Firefox
* [x] Umlaute are destroyed while loading values from database