http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31923&start=0

What si the difference for you between multilanguage and multilingual?
For me (we have the same words in French), a multilingual site in Joomla can be monolanguage.

A Mutilanguage site displays the contents and UI depending on the language chosen by the switch

I am against this change

Multilanguage is not a word in english

Multilingual has two definitions
1. Of, including, or expressed in several languages: a multilingual dictionary.
2. Using or having the ability to use several languages: a multilingual translator.

So this one word correctly expresses both a site in multiple languages and a site which can switch between multiple languages

What you do not apparently understand is that, although the word may not exist in good English Dictionnaries, it expresses a concept in Joomla!. New words make their way into all languages all over the world, specifically in the scientiffic and net business.
The term plugin (or plug-in) has only recently made its way into common usage.

Deciding suddenly after many years of use that it is wrong does not help anybody and breaks existing documentation.