Language selecter not being displayed in content creation forms

My language selector drop-down menus are no longer being displayed in content creation/editing forms. Inspecting them with firebug I can see that they are still being loaded as form items (but are grayed out - see attached screenshot). I first discovered this when trying to add a taxonomy term (to a vocab configured with 'Per language terms' and already having a number of english and french terms in it, created some time ago). Then noticed that the language selector didn't show up when I edited the vocabulary itself, nor is it showing up on any of my node creation/editing forms for content types with multilanguage support enabled.

The site is running D6.22 with i18n 6.x-1.9. Language selectors were definitely showing up in the past but I haven't personally added/edited content for a while (i.e. through a couple updates) but haven't heard any complaints from any site managers either. I have never encountered this before and am not aware of any settings that would control do this. Any help or ideas would be much appreciated!