Language switch and language subdomain.

I need WPMU installation multilanguage.

i'd like the way of
when i select a language i'm redirected to a language subdomain
ex. (main blog/default language italian) -> french blog language -> deutch blog language -> esperanto blog language

for switch language i think put a simple select into my theme, that redirect to the appropriate language subdomain.
but... how i must do to set appropriate wp_lang when i visit language subdomain?

i must create really language subdomain? or they really don't exist but are only remapping?

there is an appropriate plugin or i must change the core of WPMU?

thanks for your help