I now have the following:

add_filter( 'fep_eb_email_legends', 'fep_cus_fep_email_legends', 10, 3);

function fep_cus_fep_email_legends( $legends, $post, $user_email ){

    $user = get_user_by( 'email', $user_email );

    $language = get_user_meta( (int)$user->ID, 'icl_admin_language', true );

    $legends['receiver_language'] = array(
        'description' => __('Receiver language', 'front-end-pm'),
        'where' => array( 'newmessage', 'reply', 'announcement' ), //where this tag will be used
        'replace_with' => $language
    );

    return $legends;
}

add_filter( 'fep_eb_templates', 'fep_cus_fep_eb_templates', 10, 2 );

function fep_cus_fep_eb_templates( $templates, $where )
{
  $templates['new_message'] = 'Nieuw bericht';
  return $templates;
}

In my template I have the language by the added field receiver_language.

But how can I use this in my template in an if structure?

Is this the way I should do this?

Thank you! Works perfect! Do you also know how I can change the subject of the mail? So it’s translated?

I’ve tried this:

add_filter( 'fep_filter_before_email_send', 'change_email_subject', 10, 3 );

function change_email_subject($content, $post, $user_email)
{
  $current_language = apply_filters( 'wpml_current_language', NULL );

  $language = get_user_meta( fep_get_userdata( $user_email, 'ID', 'email'), 'icl_admin_language', true );

  // set language of user
  do_action( 'wpml_switch_language', $language );

  $subject = "";

  if($post->post_type == "fep_message")
  {
      $subject = sprintf( __('ZOUTE GRAND PRIX®: New message', 'zoute_theme' ));
  }

  $content['subject'] = $subject;

  do_action( 'wpml_switch_language', $current_language );

  return $content;
}

When I try to dump the $content before returning it, the subject is changed correctly. The content of the message itself not (but is changed afterwards with template in /emails folder).

But when I receive the email, the subject is not changed. It’s overwritten by the subject configured in backoffice.

Why doesn’t it work?