Could you try again with the development version of Polylang (should be 1.0dev17 now)?
To explain a bit about the internal of Polylang. It does load either admin or frontend and doing a wp-cron was not directed to the admin side (which means that the save_post filter used by Polylang was not called and thus the language was not added). The new dev version now directs to admin side when doing wp-cron. Hopefully this should solve your issue.
As Polylang does not provide any other way to set the language than the current WordPress admin interface, the language should be set to the default language.
More... The language is a taxonomy, but I use helper functions.
global $polylang;
$polylang->set_post_language($post_id, $slug);will set the language (defined by $slug, typically 'en', 'fr' or 'de'...) of the post defined by $post_id. Maybe it's what you need to set a different language in the postie_post filter.
