Setting frontend locale based on postmeta not loading translation

Question

I'm trying to show different language on front end based on page post meta setting. Mine problem is that I can see that locale is changed, but text still stays in English. Only if I change locale trough 'Settings > General' and set it to desired one, my translations load up and display correctly.

add_filter( 'locale', 'set_my_locale' );
function set_my_locale( $lang ) {

  if ( get_field('page_language', get_the_ID() ) ) {

    $lang = get_field('page_language', get_the_ID() );
  }

  return $lang;
}

function load_textdomain(){
  $domain = 'textdomain';

  load_theme_textdomain( $domain, trailingslashit( WP_LANG_DIR ) . $domain );
  load_theme_textdomain( $domain, get_stylesheet_directory() . '/lang' );
  load_theme_textdomain( $domain, get_template_directory() . '/lang' );
}
add_action( 'after_setup_theme', 'load_textdomain' );

If I try to print_r( get_locale() ); I can see that correct locale is set, but translations are not displayed, but instead I see original text.

If someone can point me please?

Should I use some different hook, or what I'm doing wrong?

Answer 1

Ok, I'v found out a way to do it with query_var. Looks like the problem was that locale was set to late in loading process.

If meta field/value exists redirect to page with ?lang=lang-attribute, and then set locale based on that.

function si_update_locale( $lang ) {

  if ( isset( $_GET['lang'] ) && ! empty( $_GET['lang'] ) ) {

    if( $_GET['lang'] == 'it' ) {
        $lang = 'it_IT';    
    }

  }
  return $lang;
}
add_filter( 'locale', 'si_update_locale' );


function si_load_textdomain(){

  $domain = 'textdomain';

  load_theme_textdomain( $domain, trailingslashit( WP_LANG_DIR ) . $domain );
  load_theme_textdomain( $domain, get_stylesheet_directory() . '/lang' );
  load_theme_textdomain( $domain, get_template_directory() . '/lang' );
}
add_action( 'after_setup_theme', 'si_load_textdomain' );


function si_set_locale() {
    $post_id = get_the_ID();
    $lang    = get_field('page_language', $post_id );

    if ( $lang && ! isset( $_GET['lang'] ) ) {

        switch ( $lang ) {

            case 'it_IT':

                wp_redirect( add_query_arg( array( 'lang' => 'it'), get_permalink(  $post_id ) ) );
                exit;

            break;
        }

  }
}
add_filter( 'template_redirect', 'si_set_locale' );