Yup, at least my eClicto (rebranded Netronix EB600) supports it.
Here's a test (file 1 generated with calibre 0.6.37, second one is first one verbatim copy, except the dc:language in content.opf manually changed):
With
Code:
<dc:language>UND</dc:language>
:
With
Code:
<dc:language>pl-PL</dc:language>
:
Having all the GUI options for that (setting the default language of book, changing each book's language, etc.) would be awesome.