Since Mozilla started to see the importance of localization, the number of localizations has increased ever since. Of course they have been debating about the necessity to see if it was worthwhile.
It's really important if you ask me and after what I heard this week I am convinced. Here's the small note I'd like to share:
After the release of Thunderbird 3 beta 2 we made a pressrelease, that it was also becoming available in Frisian. Somehow (maybe accidently) it showed up in a Dutch newspaper, DePers, in the Frisian language (Friesland is a province of The Netherlands). This curious thing was being noticed by so many people that it was kind of "talk of the day" in some offices. Also including the local government Fryslân. From my localization co-worker I heard that the province government now was thinking of trying to implement the use of Thunderbird! So there's your influence of a Localization.
Let's hope Thunderbird will continue to evolve and can be used in larger offices with the ability to connect properly to collaboration services. On MozCamp I heard there is some development going on again for OpenExchange and Thunderbird.