docs: add German README translation (README_DE.md) (#2994)

* docs: add German README translation (README_DE.md)

Adds README_DE.md, a full German translation modelled on README_JA.md,
and extends the language switcher in README.md, README_ZH.md and
README_JA.md with a Deutsch link.

* docs: remove outdated German sponsor entry

- Remove the AICoding.sh sponsor block from README_DE.md.
- Keep the German README aligned with the current sponsor list.

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Co-authored-by: Jason <farion1231@gmail.com>