<?php

$lang['tr'] = array
(
    'title' => "Merhaba",
    'Subtitle' => "Naber"
);

$lang['en'] = array
(
    'title' => "Hello",
    'Subtitle' => "What's Up"
);

$standart = "tr";

if (isset($_SESSION['lang'])) {
    $lang = $lang[$_SESSION['lang']];
} else {
    $lang = $lang[$standart];
}

echo $lang['title'];