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>

www/index.html

@@ -11,8 +11,9 @@
 </head>
 <body>
   <canvas id="glcanvas" tabindex="1" oncontextmenu="return false"></canvas>
-  <script>var register_plugin; /* vom quad_net-Plugin im Bundle erwartet */</script>
+  <script>var register_plugin; /* expected by the quad_net plugin in the bundle */</script>
   <script src="mq_js_bundle.js"></script>
+  <script src="dc_plugin.js"></script>
   <script>
     document.getElementById("glcanvas").focus();
     load("dungeoncraft.wasm");