Add german translations

[Grijnak]

The messages for mustBeHigherThan and mustBeLowerThan could be improved somewhat by being able to add the date in the middle i.e.

    mustBeHigherThan: (date: Date) => `Muss nach dem ${date} sein`,
    mustBeLowerThan: (date: Date) => `Muss vor dem ${date} sein`,

But it's a rather minute improvement that wouldn't be worth it if it means a lot of effort in the code.

[RichardLindhout]

I'll add custom formatting!