Enhancement: Provide informal German translations

The community was maintaining an informal German translation for years but we were only able to
provide a single version of German in the client.
We now ship both versions, the informal can be selected in the combobox in the advanced settings.
To be able to distinguish between formal and informal locales, we also include the locale
identifier in the dropdown (e.g., "Deutsch (de-informal)").

https://github.com/owncloud/client/issues/9460
https://github.com/owncloud/client/pull/9502
