We found the issue: Post Indexer plugin needed an update, to work properly with updated wordpress version (and polylang perhaps..). That’s it!
Ok, we got this solved using this part of code (found when backtracking polylang code):
$lang_code = 'en'; // this way we force en for instance (here you can pass data from user's registration for example)
global $polylang;
$mo = new PLL_MO();
$mo->import_from_db( $polylang->model->get_language( $lang_code ) );
$GLOBALS['l10n']['pll_string'] = &$mo;
$polylang->curlang = $polylang->model->get_language($lang_code);
Thanks, I figured it out already 🙂 You dont even need acf REST here, you can do it using normal wp/v2/your_post_type_here.
