Things have changed since that time. Now you should find an option in Polylang settings to activate the language and translation management for this post type. It should work if the action is "hooked to an init action".
In the code it should look like this:
add_action('init', 'bnk_register_post_type_slides');So if you can't find the checkbox in Polylang settings, try finding where 'bnk_register_post_type_slides' elsewhere in the theme's code.
Finally, I believe that's not a good idea to write:
register_post_type(__( 'slide','bhinneka' ),$args);It is better not to enable translation for something which is never displayed but used internally by WordPress. It can work on a monolingual site, but it will likely break in a multilingual site. You should change this to:
register_post_type('slide', $args);