Feature: Add German (de_DE) language support

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

server/2015Remote/LangManager.h

@@ -703,6 +703,7 @@ protected:
         if (langCode == _T("zh_CN")) return _T("Simplified Chinese");
         if (langCode == _T("zh_TW")) return _T("Traditional Chinese");
         if (langCode == _T("en_US")) return _T("English");
+        if (langCode == _T("de_DE")) return _T("Deutsch");
         return langCode;
     }