Can you tell me a bit more about the issue?
Do you mean that the drop-down list only shows items from a specific language?
Or does the actual filtering not work as intended?
It is good to know that we don’t have a specific integration for Polylang and the way they filter items based on the selected language.

The filter dropdowns get their options through an Ajax call. If you inspect the Ajax call in your browser console, you’ll see the results in JSON format on a new page. You can also use a JSON viewer extension in your browser for a clearer look at all the options.

To customize the options retrieved, we use a get_post query. According to Polylang’s documentation, you can include a ‘language’ parameter to specify the language for the query. Here’s the documentation page for reference: Polylang Developers How-To.

The Ajax call typically looks like this:
/wp-admin/admin-ajax.php?action=acp-filtering-comparison-request&layout=65f9a7d3a5826&searchterm=&column=2&list_screen=page&_ajax_nonce=f0ea30752f

To modify the query, you can utilize the ‘pre_get_posts’ hook in WordPress. You’d trigger this hook when our Ajax call is made, checking for the specific action, like:

action = acp-filtering-comparison-request

Could you give this a try and implement the hooks based on what I’ve described? While I can guide you, writing the entire code snippet isn’t something we can provide as part of our support.