Use value from Ajax call in PHP function

Question

I have a ajax script that will take a value from a select from on onchange. This works well, and with console.log I can see the correct value. My select form triggers a reload, and I see the value in console before the page reloads.

I want to use this value to set a backend language for WPML, but unsure how to use the value.

PHP

function my_action( ) {
 $dropdown_shop_order_language = $_POST['dropdown_shop_order_language'];
      
 echo json_encode($dropdown_shop_order_language);
 wp_die();

}
add_action('wp_ajax_my_action', 'my_action');
add_action('wp_ajax_nopriv_my_action', 'my_action');

This will give me a value of de, en or nl

Example of function to change language:

add_action('wp_loaded', 'my_icl_set_current_language'); 
function my_icl_set_current_language() {
    global $sitepress;
    $sitepress->switch_lang('de');
}

something like this is what Im after:

 $sitepress->switch_lang($dropdown_shop_order_language);

Whats the best approach here?

This doesnt work:

add_action('wp_loaded', 'my_icl_set_current_language'); 
function my_icl_set_current_language() {
    global $sitepress;
    $sitepress->switch_lang($dropdown_shop_order_language);
}

Answer 1

The main problem with what you're attempting is that variables do not persist across requests. If you set a variable on the AJAX request, that variable will not be set on the request to reload the page.

The other problem is the way you're referring to this as an "Example of function to change language":

add_action('wp_loaded', 'my_icl_set_current_language'); 
function my_icl_set_current_language() {
    global $sitepress;
    $sitepress->switch_lang($dropdown_shop_order_language);
}

There's 3 things here:

1. A function, my_icl_set_current_language().
2. The code inside the function that changes the language.
3. A call to add_action() which tells WordPress when to run the function.

You only need #2, and you just need to put it inside your AJAX callback:

function my_action( ) {
    global $sitepress;
    $dropdown_shop_order_language = $_POST['dropdown_shop_order_language'];

    $sitepress->switch_lang($dropdown_shop_order_language);
}
add_action('wp_ajax_my_action', 'my_action');
add_action('wp_ajax_nopriv_my_action', 'my_action');