Problem? it just save the producer :S as soon as I select the region,
and I try to update, the selected option becomes unselected again.
The underlying problem seems to be that you’re adding two select boxes on the edit page with the same name attribute, namely parent_id.
Note that this is also the same name attribute used by the default parent select box in the Page Attribute meta-box.
The parent_id is stored in the wp_posts table and that field is unsigned bigint(20). So this can only store a single integer.
You should use two different name attributes, like:
wp_dropdown_pages( [ 'name' => 'wpse_producer_id', ...
wp_dropdown_pages( [ 'name' => 'wpse_region_id', ...
where we prefixed it, with wpse_ as an example, to avoid possible name collisions.
Then you could store these values as post meta, for each post, with the help of update_post_meta(). There are some examples here in the Codex.
You would also have to adjust the selected attribute with the help of get_post_meta().
Hope it helps.