The presence and order of the names in OS X's System Preferences > International > Languages pane determines things like system and application localization, font priority, list sorting, and the Mail encoding menu. Normally by default it contains the available system localizations (18 in Leopard), but you can easily add to this by using the Edit button.
The total number of languages you can select depends to some extent on whether you have installed any extra fonts, but even with those provided by Apple it is remarkable, well over 100. I had 110 on Tiger and in Leopard this has increased to 138. A few of the more unusual entries are Klingon, Latin, Navajo, and Sanskrit. If you have a look for yourself, you will see that the languages appear in their own script, but mousing over a name gives you the English translation. You can see a full list here.