How to make Login use the language of the contaxt ?

Hi all,

I have two contexts, "web"(french version) and "english".
Their language cultureKey is set to : "fr" and "en".

Login is always using the "en" lexicon, even in the "web" context.
I don’t know how to tell Login to use French to display every page of "web" context.

Could you help me please ?

Thanks.