I'm going to use a wider definition of CMS, one that includes Wordpress and wikis. Over the years I played with a wide range of CMS systems including Drupal, Joomla, Wordpress, MediaWiki to implement multilanguage websites.
I did not find a proper CMS product that is able to deal with partial translated websites.
The biggest problem that I found so far was the one regarding the internal-links.
A normal webpage contain a number of internal links and when the translator is doing his job he is not able to change their URLs because he does not know if the targeted page is going to be translated or not (or what is going to be the localized URL of this page).
Example:
How about visiting [[en/products]] page for more details.
The translator does not know if the en/products page will be localized or not. So the only solution that will make sense is to let the CMS deal with link localization because the CMS should know if a specific page was translated and what is the proper URL. Do you know one? (only open-source)
The second issue (SEO) appears due to content duplication in several pages: we don't want to get penalized because we have the same content on different URLs /lang/page-name for all not-yet-localized pages.
You could say that we are going to link directly to the untranslated page but this does not work because once the fr visitor gets to the /en/page he will not be able to see the navigation menus in his language and also the links will go only to en pages.
3rd issue will be how to limit the search
So what would be the proper approach on solving this issue?