Thing we want here is - every time when the user switch to another language, we should update the root panel.  By tracing the `ready()` function in `l10n.js`, 
```
      if (navigator.mozL10n.ctx.isReady) {
        window.setTimeout(callback);
      } else {
        navigator.mozL10n.ctx.addEventListener('ready', callback);
      }
```
I feel it's not safe if we only rely on the `navigator.mozL10n.ready()` here because if `navigator.mozL10n.ctx.isReady` is true at this moment , then it won't register this callback on `ready` event. 
Could we keep the original code intact?