If the whole page, including navigation etc, should be in a different language, then the translation plugin should be the best and that would also set the lang attribute on the html element accordingly as well as translate the interface.
If only the main content should be in a different language, and that seems to be what you describe, then you could use the
Wrap plugin to wrap the whole content in e.g.
<WRAP :fr>
# Bonjour
(French content)
</WRAP>
That might have some unwanted side effects, though (e.g. section edit buttons might not appear).
Alternatively, it would be possible to add some kind of meta data to the page to manipulate a lang attribute on the existing content div. But I don't think any existing plugin does that so far.