bpo-31146: Don't fallback switcher to english on not-yet pusblished languages. (GH-10558)

(cherry picked from commit 6b73bb52)
Co-authored-by: Julien Palard <julien@palard.fr>

Doc/tools/static/switchers.js

@@ -48,6 +48,12 @@
       else
         buf.push('<option value="' + language + '">' + title + '</option>');
     });
+    if (!(current_language in all_languages)) {
+        // In case we're browsing a language that is not yet in all_languages.
+        buf.push('<option value="' + current_language + '" selected="selected">' +
+                 current_language + '</option>');
+        all_languages[current_language] = current_language;
+    }
     buf.push('</select>');
     return buf.join('');
   }