Reply To: Plugin bug in multilanguage site (maybe).

I use the normal function from the wordpress codex to load the language. It uses get_locale(); to find out which language is active. If you’ve set WP_LANG is it_IT it takes the italian language file as default.
I don’t know polylang, but if it doesn’t change the result of get_locale() as of the users select I wouldn’t recommend to use it.