diff --git a/simulator/static/app.js b/simulator/static/app.js
index 8bf34a1..8c0b7e9 100644
--- a/simulator/static/app.js
+++ b/simulator/static/app.js
@@ -53,6 +53,8 @@ let currentClipId = null;  // base media currently loaded (reset to force a relo
 let busy = false;          // true while a ring transition is playing
 let morphByPair = {};      // "fromClip→toClip" -> directed morph file (built from the ring)
 let lastOverlay = null;    // {level, intensity} from the most-recent renderOverlay call; drives per-frame track animation
+let activeLang = "en";     // session-only; no persistence (resets to en each load)
+let lastAffect = null;     // {strength, intensity, right} from the last renderAffect, for re-render on language switch
 
 async function loadData() {
   const clips = (await (await fetch("/api/clips")).json()).clips || [];
@@ -502,7 +504,7 @@ function renderOverlay(level, intensity) {
   overlay.innerHTML = "";
   if (!clip || level <= 0) { overlay.style.opacity = "0"; return; }
   overlay.style.opacity = String(intensity);
-  const strings = (clip.strings && clip.strings.en) || {};
+  const strings = HEFi18n.resolveStrings(clip.strings, activeLang);
   const t = loopT();
   let shown = 0;
   for (const a of clip.annotations) {
@@ -548,12 +550,13 @@ function renderOverlay(level, intensity) {
 // scene points (no boxes — feelings are scene-level) and read softer than the
 // clinical reticles — feeling, not measurement.
 function renderAffect(strength, intensity, right) {
+  lastAffect = { strength, intensity, right };
   const clip = activeClip();
   if (!affectLayer) return;      // tolerate a stale page missing the affect layer
   affectLayer.innerHTML = "";
   if (!clip || !clip.affect || strength <= 0) { affectLayer.style.opacity = "0"; return; }
   affectLayer.style.opacity = String(intensity);
-  const strings = (clip.strings && clip.strings.en) || {};
+  const strings = HEFi18n.resolveStrings(clip.strings, activeLang);
   for (const f of clip.affect) {
     if (f.min_level > strength) continue;
     const [x, y] = f.at.map((n) => n * 100);
@@ -572,7 +575,8 @@ function renderScaleReadout() {
   const member = (activeClip() && activeClip().title) || s.title;
   // scale id · the chosen pool member · position on the ring (pool size if >1)
   const poolTag = poolN > 1 ? ` · pool ${poolN}` : "";
-  $("scale-name").textContent = `${s.id} · ${member}  (${ringIndex + 1}/${ring.scales.length}${poolTag})`;
+  const scaleName = HEFi18n.pickUiString("scale." + s.id, activeLang);
+  $("scale-name").textContent = `${scaleName} · ${member}  (${ringIndex + 1}/${ring.scales.length}${poolTag})`;
   renderDial();
   refreshDevClip();      // keep the Dev Mode pool picker + clip data in sync with the landing
 }
@@ -1218,6 +1222,7 @@ async function main() {
   await landScale();          // pick the initial scale's pool member before first render
   buildDial();                // draw the altitude knob from the ring's scales
   initDev();                  // wire the Dev Mode toggle + pool picker (reads persisted state)
+  initLanguage();             // populate the language dropdown + wire live switching
   renderScaleReadout();
   // Sliders stream on "input"; the toggles fire on "change".
   for (const id of ["left", "right", "mood"]) $(id).addEventListener("input", debounced);
@@ -1248,6 +1253,39 @@ async function main() {
   autoStart();                // once loaded: video on + gentle audio (3/10), no gesture needed
 }
 
+// Populate the language dropdown from the registry and wire live switching.
+// English default, session-only (no persistence). Switching swaps UI chrome
+// and re-renders the current annotations/affect in place — no reload, no fetch.
+function initLanguage() {
+  const sel = $("lang-select");
+  if (!sel) return;
+  for (const { code, nativeName } of HEFi18n.LANGUAGES) {
+    const o = document.createElement("option");
+    o.value = code;
+    o.textContent = nativeName;
+    sel.appendChild(o);
+  }
+  sel.value = activeLang;
+  applyUiStrings(activeLang);
+  sel.addEventListener("change", () => setLanguage(sel.value));
+}
+
+// Fill every [data-i18n] node with its catalog string for `lang`.
+function applyUiStrings(lang) {
+  document.documentElement.lang = lang;
+  for (const el of document.querySelectorAll("[data-i18n]")) {
+    el.textContent = HEFi18n.pickUiString(el.getAttribute("data-i18n"), lang);
+  }
+}
+
+function setLanguage(lang) {
+  activeLang = lang;
+  applyUiStrings(lang);
+  renderScaleReadout();
+  if (lastOverlay) renderOverlay(lastOverlay.level, lastOverlay.intensity);
+  if (lastAffect) renderAffect(lastAffect.strength, lastAffect.intensity, lastAffect.right);
+}
+
 // Auto-start the experience the moment media is ready: bring Video on and the audio
 // level to 3/10 (the same gentle default the first manual Video-on couples in). The
 // audio <audio> elements ideally unlock inside a user gesture (Safari); this fires