diff --git a/simulator/static/index.html b/simulator/static/index.html
index f790bde..ce60563 100644
--- a/simulator/static/index.html
+++ b/simulator/static/index.html
@@ -9,11 +9,11 @@
 <body>
   <div id="loading">
     <div class="loading-inner">
-      <div class="loading-title">Loading Universe<span class="loading-dots"></span></div>
+      <div class="loading-title"><span data-i18n="loading.title">Loading Universe</span><span class="loading-dots"></span></div>
       <div class="loading-bar"><div id="loading-fill"></div></div>
     </div>
   </div>
-  <header><h1>Human Experience Filter — Alteration Preview</h1></header>
+  <header><h1 data-i18n="app.title">Human Experience Filter — Alteration Preview</h1></header>
   <main>
     <section class="stage" id="stage">
       <div class="screen">
@@ -31,39 +31,42 @@
 
     <section class="panel">
       <fieldset>
-        <legend>Output</legend>
+        <legend data-i18n="output.legend">Output</legend>
+        <label class="lang-pick">🌐
+          <select id="lang-select" aria-label="Language"></select>
+        </label>
         <label class="dev-switch" for="visual">
           <input type="checkbox" id="visual" />
           <span class="dev-switch-track"><span class="dev-switch-thumb"></span></span>
-          <span class="dev-switch-label">Video</span>
+          <span class="dev-switch-label" data-i18n="output.video">Video</span>
         </label>
-        <label class="audio-level">Audio
+        <label class="audio-level"><span data-i18n="output.audio">Audio</span>
           <input type="range" id="audio" min="0" max="10" value="0" step="1" />
           <span id="audio-level-val">0</span>/10
         </label>
       </fieldset>
 
       <fieldset>
-        <legend>Altitude</legend>
+        <legend data-i18n="altitude.legend">Altitude</legend>
         <div class="dial-wrap">
           <svg id="dial" viewBox="0 0 100 100" aria-label="Altitude knob (turn to change scale)"></svg>
         </div>
         <span id="scale-name" class="scale-name">—</span>
-        <p class="hint">Turn the knob (drag it, or scroll) to change altitude — endless: past the deepest it wraps back up to the highest. Click a label to jump there.</p>
+        <p class="hint" data-i18n="altitude.hint">Turn the knob (drag it, or scroll) to change altitude — endless: past the deepest it wraps back up to the highest. Click a label to jump there.</p>
       </fieldset>
 
       <fieldset>
-        <legend>Experience knobs (0–4)</legend>
-        <label>Think <input type="range" id="left" min="0" max="4" value="0" /></label>
-        <label>Feel <input type="range" id="right" min="0" max="4" value="0" /></label>
-        <label>Mood — dark ◀ 0 ▶ light <input type="range" id="mood" min="-4" max="4" value="0" /></label>
+        <legend data-i18n="knobs.legend">Experience knobs (0–4)</legend>
+        <label><span data-i18n="knobs.think">Think</span> <input type="range" id="left" min="0" max="4" value="0" /></label>
+        <label><span data-i18n="knobs.feel">Feel</span> <input type="range" id="right" min="0" max="4" value="0" /></label>
+        <label><span data-i18n="knobs.mood">Mood — dark ◀ 0 ▶ light</span> <input type="range" id="mood" min="-4" max="4" value="0" /></label>
       </fieldset>
 
       <!-- Dev Mode: a single toggle; everything dev (pool picker + analysis) lives below it. -->
       <label class="dev-switch" for="dev-mode">
         <input type="checkbox" id="dev-mode" />
         <span class="dev-switch-track"><span class="dev-switch-thumb"></span></span>
-        <span class="dev-switch-label">Dev Mode</span>
+        <span class="dev-switch-label" data-i18n="devmode.label">Dev Mode</span>
       </label>
 
       <div id="dev-panel" class="dev-panel hidden">
@@ -105,6 +108,7 @@
     </section>
   </main>
   <script src="/scrub.js"></script>
+  <script src="/i18n.js"></script>
   <script src="/app.js"></script>
 </body>
 </html>
diff --git a/simulator/unit/index-i18n.test.js b/simulator/unit/index-i18n.test.js
new file mode 100644
index 0000000..345c3c2
--- /dev/null
+++ b/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}`);
+});
