German translation: Initial translation (draft)

[janezd]

Add German translation, one German-translation-related fix, and a fixed type in Slovenian translation.

Possible problems (among others):

• Bad translations, e.g. bad wording, a different word used in translation.
• Incorrect translations of code in f-strings, e.g. translated variable names, incorrect calls of plde.
• "Strings" that must not be translated. These were marked as "must not be translated" in the Slovenian translation, which is used as template, but I could have missed some. E.g. in open(..., error="strict"), I did't mark "strict" as non-translatable, and in the German translation it was translated to "strikt".

The first problem causes embarrasment, the latter two cause runtime errors.