Adds the German locale per maintainer guidance in issue #1980.
- docs/de-DE/README.md: full translation of the English root README
- docs/de-DE/GLOSSARY.md: German terminology decisions (which ECC
terms stay English vs. German)
- manifests: docs-de-de module + locale:de-de component, modelled on
the existing docs-zh-tw / locale:zh-tw entries
- scripts/lib/install-manifests.js: de-DE in SUPPORTED_LOCALES, plus
de-DE and de aliases so `--locale de` and `--locale de-DE` resolve
- tests/lib/locale-install.test.js: focused de-DE coverage
- Deutsch added to the language selector tables in all 10 existing
READMEs (root + 9 localized)
Refs #1980
Martin Klein