WebDriver BiDi: override Accept-Language header

Tests: web-platform-tests/wpt#55977.

Author: lutien

fetch.bs

@@ -4663,6 +4663,27 @@ the response. [[!HTTP-CACHING]]
    <var>request</var>'s <a for=request>header list</a>.
   </ol>
 
+ <li>
+  <p>If <var>request</var>'s <a for=request>header list</a>
+  <a for="header list">does not contain</a> `<code>Accept-Language</code>` and <var>request</var>'s
+  <a for=request>client</a> is non-null:
+
+  <ol>
+   <li><p>Let <var>emulatedLanguage</var> be the <a>WebDriver BiDi emulated language</a> for
+   <var>request</var>'s <a for=request>client</a>.
+
+   <li><p>If <var>emulatedLanguage</var> is non-null:
+
+   <ol>
+    <li><p>Let <var>encodedEmulatedLanguage</var> be <var>emulatedLanguage</var>,
+    <a>isomorphic encoded</a>.
+
+    <li><p><a for="header list">Append</a>
+    (`<code>Accept-Language</code>`, <var>encodedEmulatedLanguage</var>) to <var>request</var>'s
+    <a for=request>header list</a>.
+   </ol>
+  </ol>
+
  <li><p>If <var>request</var>'s <a for=request>header list</a>
  <a for="header list">does not contain</a> `<code>Accept-Language</code>`, then user agents should
  <a for="header list">append</a> (`<code>Accept-Language</code>, an appropriate