more than one natural languages in atom xml

Question

I have web pages with versions in french and english. My server use the html language negotiation to deliver the right page based on the browser preference. This works well, the same URL will show up either in english or french based on your browser setting.

I have decided to add an atom feed for some pages. Is there a way to give the summary in more than one language? I have tried both this forms, and none validate:

<summary xml:lang="en">some text here</summary>
<summary xml:lang="fr">du texte ici</summary>

(says there are two summaries, only one is valid)

and

<summary xml:lang="en">some text here <xml:lang="fr">du texte ici></summary>

(says summary is malformed)

Thanks.