Add language picker to the cli and the ui

[TheOneRing]

Implement a language picker to the ci.
The selected language is stored in the settings and loaded at start up.
Additionally add a --language switch to the command line interface which temporarily overrides the selection.

[sfadschm]

Just to add to that: It would be nice if the localization of the language was choosable in that case also.
E.g., the client_de translation is currently based on the de_DE dataset, but there also exist a de, de_AT.

[fmoc]

Adding --language was relatively easy. I'm about a language dropdown to the settings at the moment, but first I need to find a good way to detect the available translations first.

Regarding the CLI, it doesn't seem like it's being translated at the moment. I didn't find any code that would integrate translations in there. @TheOneRing I guess the CLI bit needs to be moved into a new issue, given that it's way out of scope of this issue.