WPML - REST API

functions.php

/**
 * Handle WPML REST requests
 */
add_action('rest_api_init', function() {

    if (isset($_REQUEST['lang']) && !empty($_REQUEST['lang'])) {
        global $sitepress;
        $availableLangs = array_keys(apply_filters('wpml_active_languages', []));
        $currentLang = get_query_var('lang');
        $newLang = sanitize_text_field($_REQUEST['lang']);

        if ($newLang !== $currentLang && in_array($newLang, $availableLangs)) {
            $sitepress->switch_lang($newLang);
        }
    }
});