* 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* docs: remove outdated German sponsor entry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.
---------
Co-authored-by: Jason <farion1231@gmail.com>
Martin Klein