[Zope] changing default language

[Samuele Giovanni Tonon]

hi to all,
i'm playing with i18n:translate tag of ZPT.

I would like a way to force i18n to show me the right language which is 
NOT setted on the browser, but on a cookie setted by a user preference
interface.

I tried some way to do that but none worked; i think that there can be
only two solutions to that :
Find a way to overwrite the default language of the browser (is it 
possible ? or it's something that has to be done only on to the browser ? )

Find a way to tell i18n:translate="" to change to i18n:translate="lang"
(where lang is a tal variable that stores the language user preference); 
however i was not able to find how to do that evaluation of lang and
i18n:target="string:something" seems not to work.

Any other solutions ? Any hints ?

Many thanks
Samuele