Following on from a question at FOSDEM, the following localisations are on the 1.6 CD:

Chinese, Traditional - langzhtw.xpi
English, Great Britain - langengb.xpi
Estonian - langet.xpi
French - langfrfr.xpi
Galician - langgles.xpi
German, Austrian - langdeat.xpi
Greek - langelgr.xpi
Italian - langitit.xpi
Japanese - langjajp.xpi
Korean - langkokr.xpi
Norwegian Nynorsk - langnnno.xpi
Norwegian Bokmål - langnbno.xpi
Polish - langplpl.xpi
Slovene - langslsi.xpi
Spanish Language - langeses.xpi
(Spanish Region - reges.xpi)

Total: 15.

Going on the rough Google Zeitgeist figures, this covers the language of at least 90% of all web users.

Gerv