There are some XLIFF translation products
http://svn.plone.org/svn/collective/Products.XLIFFMarshall/trunk/Products/XLIFFMarshall/
https://svn.plone.org/svn/collective/slc.xliff/trunk/slc/xliff/
What would be the best way of import / export HTML'ish content from a site for translation or would it be better to let translators to have user accounts? We have had bad experience letting translators to go to edit the site itself, as they need quite high priviledges for example, translating the front page.
I think it depends on the translators and the translation workflow.
If the translation is outsourced to one or several agencies, which internally distribute work, XLIFF is a really good idea. Generally you cannot expect the translators to learn about Plone here. EEA (http://www.eea.europa.eu/) is one example of using this approach.
If you have a fixed number of freelance or in-house translators, you could train them to use Plone. But it depends on their technical ability, if you want to give them editor rights to all parts of the site or not. The Nordic Council (http://www.norden.org/) uses this second approach.
So I'd say this isn't a technical problem, but one where you need to choose the right tool/workflow for the specific situation.