uutils · sylvestre · Aug 31, 2026 · Aug 31, 2026
diff --git a/content/playground-how-it-works.md b/content/playground-how-it-works.md
@@ -119,7 +119,10 @@ flowchart TB
     G --> I["uutils.wasm<br/>(compileStreaming)"]
     H --> J["Ready"]
     I --> J
-    J --> K{"?cmd= parameter?"}
+    J --> K0{"?lang= parameter?"}
+    K0 -->|Yes| K1["Set locale (LANG=…)"]
+    K1 --> K{"?cmd= parameter?"}
+    K0 -->|No| K
     K -->|Yes| L["Auto-run commands"]
     K -->|No| M["Show prompt"]
 </pre>

diff --git a/content/playground.md b/content/playground.md
@@ -112,6 +112,15 @@ Multiple commands can be run in sequence, separated either by `;` on one line or
 - [`?cmd=echo hello%0Aecho world`](/playground?cmd=echo%20hello%0Aecho%20world) - run two commands in sequence
 - [`?cmd=updatedb; locate names`](/playground?cmd=updatedb%3B%20locate%20names) - build the locate database, then search it
 
+Add `?lang=` to run the command in another language - it sets the locale the
+same way the **Language** dropdown does, before the command runs. Both the full
+form (`fr-FR`) and the short form (`fr`) work, and an unknown language is
+ignored. The share-link button includes it automatically when the locale isn't
+the default:
+
+- [`?lang=fr-FR&cmd=cut -f 1,4-2,9-12 fruits.txt`](/playground?lang=fr-FR&cmd=cut%20-f%201%2C4-2%2C9-12%20fruits.txt) - a French error message
+- [`?lang=de&cmd=ls /nope`](/playground?lang=de&cmd=ls%20%2Fnope) - a German error message
+
 ## Available commands
 
 The following commands run as **real Rust coreutils compiled to WebAssembly**:

diff --git a/static/js/playground.js b/static/js/playground.js
@@ -74,6 +74,9 @@ document.addEventListener("DOMContentLoaded", function() {
       var url = new URL(window.location.href);
       url.search = "";
       url.hash = "";
+      // Only carry the locale when it isn't the default, to keep links short.
+      var locale = window.getLocale ? window.getLocale() : "";
+      if (locale && locale !== "en-US") url.searchParams.set("lang", locale);
       url.searchParams.set("cmd", cmd);
       return url.toString();
     };
@@ -103,17 +106,34 @@ document.addEventListener("DOMContentLoaded", function() {
   }
 
   // Populate the locale dropdown from the build-generated list
-  if (typeof WASM_LOCALES !== "undefined") {
-    var sel = document.getElementById("locale-select");
+  var localeSelect = document.getElementById("locale-select");
+  if (typeof WASM_LOCALES !== "undefined" && localeSelect) {
     WASM_LOCALES.forEach(function(loc) {
       if (loc === "en-US") return; // already the default option
       var opt = document.createElement("option");
       opt.value = loc;
       opt.textContent = loc;
-      sel.appendChild(opt);
+      localeSelect.appendChild(opt);
     });
   }
 
+  // Keep the dropdown in sync when the locale is set elsewhere: the `locale`
+  // builtin, or the ?lang= URL parameter on load.
+  document.addEventListener("uutils:locale-changed", function(e) {
+    if (!localeSelect || !e.detail) return;
+    var loc = e.detail.locale;
+    var known = Array.prototype.some.call(localeSelect.options, function(o) {
+      return o.value === loc;
+    });
+    if (!known) {
+      var opt = document.createElement("option");
+      opt.value = loc;
+      opt.textContent = loc;
+      localeSelect.appendChild(opt);
+    }
+    localeSelect.value = loc;
+  });
+
   // Populate the "Available commands" list from the build-generated list
   if (typeof WASM_COMMANDS !== "undefined" && Array.isArray(WASM_COMMANDS)) {
     var listEl = document.getElementById("wasm-commands-list");

diff --git a/static/js/wasm-terminal.js b/static/js/wasm-terminal.js
@@ -563,6 +563,37 @@ function sanitizeUrlCommand(raw) {
   return cmd;
 }
 
+/**
+ * Normalize a locale name: "fr" -> "fr-FR", "en" -> "en-US", full forms as-is.
+ */
+function normalizeLocale(raw) {
+  const arg = (raw || "").trim();
+  if (!arg) return "";
+  return arg.includes("-") ? arg : LOCALE_SHORTCUTS[arg.toLowerCase()] || arg;
+}
+
+/**
+ * Sanitize a locale coming from the ?lang= URL parameter.
+ *
+ * Accepts either a shortcut ("fr") or a full locale ("fr-FR"), and only
+ * returns a value the build actually ships (WASM_LOCALES, when available) so
+ * a bogus ?lang= cannot push an arbitrary string into the LANG environment
+ * variable handed to the WASM runtime. Returns "" when unusable.
+ */
+function sanitizeUrlLocale(raw) {
+  const locale = normalizeLocale(raw);
+  if (!locale || !/^[A-Za-z]{2,3}(-[A-Za-z0-9]{2,8})?$/.test(locale)) return "";
+  const available =
+    (typeof WASM_LOCALES !== "undefined" && Array.isArray(WASM_LOCALES) && WASM_LOCALES.length > 0)
+      ? WASM_LOCALES
+      : null;
+  if (available) {
+    const match = available.find(l => l.toLowerCase() === locale.toLowerCase());
+    return match || "";
+  }
+  return locale;
+}
+
 /**
  * Read a file from the virtual filesystem. Returns its content as a string,
  * or null if not found.
@@ -698,9 +729,8 @@ async function executeSingleCommandLine(line) {
     if (!arg) {
       return `LANG=${currentLocale}.UTF-8\n`;
     }
-    // Normalize: "fr" -> "fr-FR", "en" -> "en-US", or accept full form
-    const normalized = arg.includes("-") ? arg : LOCALE_SHORTCUTS[arg.toLowerCase()] || arg;
-    currentLocale = normalized;
+    currentLocale = normalizeLocale(arg);
+    notifyLocaleChanged();
     return `Locale set to ${currentLocale}\n`;
   }
 
@@ -1112,8 +1142,18 @@ async function initPlayground(containerId) {
     terminal.writeln("Try reloading the page.");
   }
 
+  // Apply the locale from the URL ?lang= parameter before running ?cmd=, so a
+  // shared link like ?lang=fr-FR&cmd=... shows the localized output.
+  const params = new URLSearchParams(window.location.search);
+  const urlLocale = sanitizeUrlLocale(params.get("lang"));
+  if (urlLocale) {
+    currentLocale = urlLocale;
+    notifyLocaleChanged();
+    terminal.writeln(`Locale set to ${currentLocale}`);
+  }
+
   // Run command(s) from URL ?cmd= parameter if present
-  const urlCmd = sanitizeUrlCommand(new URLSearchParams(window.location.search).get("cmd"));
+  const urlCmd = sanitizeUrlCommand(params.get("cmd"));
   if (urlCmd) {
     for (const cmd of urlCmd.split("\n")) {
       if (cmd.trim()) await runInTerminal(cmd.trim());
@@ -1137,11 +1177,22 @@ async function runInTerminal(cmd) {
   prompt();
 }
 
+/**
+ * Let the page chrome (locale dropdown, share link) know the locale changed,
+ * whichever way it was set: dropdown, `locale` builtin or ?lang= URL param.
+ */
+function notifyLocaleChanged() {
+  document.dispatchEvent(new CustomEvent("uutils:locale-changed", {
+    detail: { locale: currentLocale },
+  }));
+}
+
 /**
  * Set the locale and optionally update the terminal.
  */
 function setLocale(locale) {
   currentLocale = locale;
+  notifyLocaleChanged();
   if (terminal) {
     terminal.writeln(`\r\nLocale set to ${currentLocale}`);
     prompt();
@@ -1154,6 +1205,7 @@ window.uutilsExecute = executeCommandLine;
 window.runInTerminal = runInTerminal;
 window.setLocale = setLocale;
 window.getLastCommand = () => lastCommand;
+window.getLocale = () => currentLocale;
 
 // On-demand loading of the optional standalone modules, used by the "Load"
 // buttons on the playground page. Buttons operate on groups (see