feat(i18n): tag controls with data-i18n + add language dropdown

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/unit/index-i18n.test.js

@@ -0,0 +1,19 @@
+"use strict";
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const fs = require("node:fs");
+const path = require("node:path");
+const I = require("../static/i18n.js");
+
+const html = fs.readFileSync(path.join(__dirname, "../static/index.html"), "utf8");
+
+test("index.html includes the i18n script and a language select", () => {
+  assert.match(html, /<script src="\/i18n\.js">/);
+  assert.match(html, /id="lang-select"/);
+});
+
+test("every data-i18n key in index.html exists in UI_STRINGS", () => {
+  const keys = [...html.matchAll(/data-i18n="([^"]+)"/g)].map((m) => m[1]);
+  assert.ok(keys.length >= 12, `expected many tagged nodes, found ${keys.length}`);
+  for (const k of keys) assert.ok(I.UI_STRINGS[k], `data-i18n key not in catalog: ${k}`);
+});