Part of your issue is that you have an incomplete document. It should really read:

<locale lang="en">English Part 1</locale>
<locale lang="fr">French Part 1</locale>
<locale lang="it"></locale>
<locale lang="en">English Part 2</locale>
<locale lang="fr"></locale>
<locale lang="it">Italian Part 2</locale>
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That way, you have a copy for each language in each section. Alternately, you can provide additional semantic meaning by explicating the sections, as you mentioned as your first option. However, I wouldn't make it locale-specific. I'd do something like:

<section id="part1">
  <locale lang="en">English Part 1</locale>
  <locale lang="fr">French Part 1</locale>
</section>
<section id="part2">
    <locale lang="en">English Part 2</locale>
    <locale lang="it">Italian Part 2</locale>
</section>
[download]

Now, you have a place to hang other stuff, like formatting. Don't get tunnel-vision.