Will a language translation that was updated for 4.0.5. work properly on 4.0.6+ or will it need to be updated?! I would like to add French as a language to my site but the most recent one available in the marketplace is for 4.0.5. Can some clever person please advise on this? 

You can import them without worry. There have been a few occasions when the language strings have changed but this has been rare since 4.0 was released. If you find a mistake when you're using the site, it won't take long to make a change to correct it.

This is like in v3.x versions... if you had a translations for v3.3 but you used it for v3.4 it worked fine, but you had some untranslated strings, that was the only problem... and when I say some untranslated, you must think in less than 1%

As you must think that v4.0.6 is around 9.000 strings and they have only added less than 50 new strings from v4.0.4 to v4.0.6