rowan.collins wrote:

This could be a caching issue - versions of pages with incomplete interface
translation not being cleared properly from a cache somewhere. This seems to be
the most common cause of randomly intermittent behaviours like this.

This would explain the randomness but not the untranslated interface.
There must not be any state of the page containing untranslated interface so it could not be cached any way.
And the untranslatedness is consistent: only those I mentioned stay untranslated, and always those only, but all of them. I wouldn't
blame this on any caching (tabs are part of the page and not referenced outside, right? so they couldn't be cached separately...).

(Except if somehow someone cached a version 6 months ago and burps it up now... I doubt that.)

I checked a buggy page, maybe this helps: Served by friedrich.wikimedia.org in 0.46 secs.

I guess the problem is that the software occassionally switches
from MediaWiki: messages to LanguageXx.php messages.
We have experienced similar problems on cs:, see e.g.
http://mail.wikipedia.org/pipermail/wikitech-l/2004-October/025758.html

This is an old bug that got fixed since. Feel free to reopen
it if the issue is still happening :)