Switch repo to English, add i18n with EN/DE language files and first-start language selection

- in-game texts moved to lang/en.lang and lang/de.lang (key = text, placeholders)
- game logic emits message keys + args, translated at display time
- localStorage access via own miniquad JS plugin (www/dc_plugin.js)
- code comments, DESIGN.md, CREDITS.md translated; README.md added

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nginx.conf

@@ -2,7 +2,7 @@ server {
     listen 80;
     root /usr/share/nginx/html;
     index index.html;
-    # Testversion: immer frisch laden
+    # test build: always reload fresh files
     add_header Cache-Control "no-cache";
     location / {
         try_files $uri $uri/ =404;