Add German as the fourth selectable UI language. The new de_DE.ini mirrors
en_US.ini with byte-identical Chinese keys (2028 lines, zero missing
translations, zero key mismatches), so it passes CheckEncoding and appears in
the language list automatically. German values are pure-ASCII transliterations
(ae/oe/ue/ss) because the GBK/CP936 architecture cannot encode umlauts.
LangManager.h now maps de_DE to the display name "Deutsch". Also translated the
seven Chinese value==key gaps that en_US itself left untranslated.
Co-Authored-By: deepseek-v4-pro
