Hi!
It looks like the UCP language is stored in a cookie called lang with a path of "/ucp"...
Maybe there's a way to set it by passing a parameter or something similar but I have not found it yet...
I even tried telling my browser to ask for another language by default and that didn't work...
Assuming there is not actually a way to do it the two best ways to do this I guess would be to either store the user language in the database and/or make it possible for it to be specified in the URL provided to the user.
In the mean time however I think that your best bet would be to use Apache's mod_rewrite to set a cookie (using the "cookie flag") and possibly rewrite the URL...
It kind of depends whether you want to support one or more language for the same installation... If you need to support more than one language at the same time I would use different URLs specific to each of the languages needed to be supported otherwise I would only set the cookie and not rewrite the URL.
If you only need to support one language and not rewrite the URL I guess you could also use mod_headers to set that cookie...
Best bet however would be to be able to do from FreePBX but if there is a way I have not found it yet...
Bonne chance et bonne journée!
(Good luck and have a nice day!)
Nick