diff --git a/.github/docker/ci/lib.sh b/.github/docker/ci/lib.sh
index bdd06656bc..dd122e76c5 100644
--- a/.github/docker/ci/lib.sh
+++ b/.github/docker/ci/lib.sh
@@ -88,8 +88,8 @@ http_check() {
 
     php -r '
         // The front end and the manager are the two entry points an installed
-        // site has to serve. The manager answers 404 without an Accept-Language
-        // header by design, hence the header on the second request.
+        // site has to serve. The manager request carries no Accept-Language
+        // header on purpose: the manager must fall back to English.
         $port = $argv[1];
         $expect = $argv[2];
         $get = function (string $path, array $headers = []) use ($port): array {
@@ -125,7 +125,7 @@ http_check() {
             $failures[] = "the front page rendered nothing";
         }
 
-        [$status, $body] = $get("/manager/index.php", ["Accept-Language: en-US,en;q=0.9"]);
+        [$status, $body] = $get("/manager/index.php");
         echo "manager: {$status}\n";
         $status === 200 or $failures[] = "the manager answered {$status}";
         stripos($body, "password") !== false or $failures[] = "the manager did not render its login form";
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index fb5cacc45d..b2a3cf56d4 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -114,8 +114,8 @@ jobs:
             curl -sf -o /dev/null http://127.0.0.1:8899/ && break
           done
           front=$(curl -s -o /dev/null -w '%{http_code}' http://127.0.0.1:8899/)
-          # The manager answers 404 without an Accept-Language header by design.
-          manager=$(curl -s -o /dev/null -w '%{http_code}' -H 'Accept-Language: en-US,en;q=0.9' http://127.0.0.1:8899/manager/index.php)
+          # No Accept-Language header on purpose: the manager must default to English.
+          manager=$(curl -s -o /dev/null -w '%{http_code}' http://127.0.0.1:8899/manager/index.php)
           echo "front=$front manager=$manager"
           test "$front" = "200" && test "$manager" = "200"
 
@@ -285,8 +285,8 @@ jobs:
             curl -sf -o /dev/null http://127.0.0.1:8899/ && break
           done
           front=$(curl -s -o /dev/null -w '%{http_code}' http://127.0.0.1:8899/)
-          # The manager answers 404 without an Accept-Language header by design.
-          manager=$(curl -s -o /dev/null -w '%{http_code}' -H 'Accept-Language: en-US,en;q=0.9' http://127.0.0.1:8899/manager/index.php)
+          # No Accept-Language header on purpose: the manager must default to English.
+          manager=$(curl -s -o /dev/null -w '%{http_code}' http://127.0.0.1:8899/manager/index.php)
           # A friendly URL has to be routed into index.php rather than answered
           # by the web server itself. Whether the CMS then renders a page or an
           # error is not the point — reaching PHP at all is — so this looks for
diff --git a/core/tests/Unit/Manager/ManagerAcceptLanguageDefaultTest.php b/core/tests/Unit/Manager/ManagerAcceptLanguageDefaultTest.php
new file mode 100644
index 0000000000..8b648822ca
--- /dev/null
+++ b/core/tests/Unit/Manager/ManagerAcceptLanguageDefaultTest.php
@@ -0,0 +1,41 @@
+<?php
+
+function managerIndexSource(): string
+{
+    return file_get_contents(dirname(__DIR__, 4) . '/manager/index.php');
+}
+
+function managerAcceptLanguageDefaultStatement(): string
+{
+    preg_match('/^\$_SERVER\[\'HTTP_ACCEPT_LANGUAGE\'\]\s*\?\?=\s*[^;]+;/m', managerIndexSource(), $match);
+
+    return $match[0] ?? '';
+}
+
+it('does not answer 404 when the Accept-Language header is missing', function () {
+    $source = managerIndexSource();
+
+    expect($source)->not->toMatch('/if\s*\(\s*!\s*isset\(\$_SERVER\[\'HTTP_ACCEPT_LANGUAGE\'\]\)\s*\)\s*\{\s*header\([^)]*404/');
+});
+
+it('defaults a missing Accept-Language header to English', function () {
+    $statement = managerAcceptLanguageDefaultStatement();
+    expect($statement)->not->toBe('');
+
+    $backup = $_SERVER['HTTP_ACCEPT_LANGUAGE'] ?? null;
+    try {
+        unset($_SERVER['HTTP_ACCEPT_LANGUAGE']);
+        eval($statement);
+        expect($_SERVER['HTTP_ACCEPT_LANGUAGE'])->toBe('en');
+
+        $_SERVER['HTTP_ACCEPT_LANGUAGE'] = 'de-DE,de;q=0.9';
+        eval($statement);
+        expect($_SERVER['HTTP_ACCEPT_LANGUAGE'])->toBe('de-DE,de;q=0.9');
+    } finally {
+        if ($backup === null) {
+            unset($_SERVER['HTTP_ACCEPT_LANGUAGE']);
+        } else {
+            $_SERVER['HTTP_ACCEPT_LANGUAGE'] = $backup;
+        }
+    }
+});
diff --git a/manager/index.php b/manager/index.php
index b951eddf4d..7c1ffe3c39 100755
--- a/manager/index.php
+++ b/manager/index.php
@@ -96,10 +96,8 @@
     exit;
 }
 
-if (!isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
-    header('HTTP/1.0 404 Not Found');
-    exit;
-}
+// Clients that send no Accept-Language header (API clients, health checks) get English.
+$_SERVER['HTTP_ACCEPT_LANGUAGE'] ??= 'en';
 
 // send anti caching headers
 header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');