class MyPage extends Page {
}

class MyPageController extends PageController {
    private static $allowed_actions = ['show'];
    private static $url_handlers = ['show/$ID' => 'show'];

    public function show() {
         ....
    }
}

<select class="select" onchange="window.location.href=this.value;">
    <% loop Locales %>
        <% if LinkingMode = current %>
            <option selected value="$Link.ATT">$Title.XML</option>
        <% else %>
            <option value="$Link.ATT">$Title.XML</option>
        <% end_if %>
    <% end_loop %>
</select>

<select class="select" onchange="window.location.href=this.value;">
     <option value="www.example.com/en/mypage/" selected="">English</option>
     <option value="www.example.com/ar/mypage/">العربية</option>
</select>

<select class="select" onchange="window.location.href=this.value;">
     <option value="www.example.com/en/mypage/show/5" selected="">English</option>
     <option value="www.example.com/ar/mypage/show/5">العربية</option>
</select>