How to make a site multilingual?

Question

PHP

What needs to be fixed to make the code work?
The main point is that the language on the site is set automatically, but it is possible to change it manually
if (isset($_SESSION['lang'])) {
if ($_SESSION['lang'] != "") {
preg_match('/^\ w{2}/',$_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches);
switch (strtolower($matches[0])){
case "ru": $accept_lang="ru";break;
case "en": case "uk": case "us": $accept_lang="en";break;
default: $accept_lang="en";break;
}
require_once './lang/' . $accept_lang . '.php';
if ($_SESSION['lang'] == "ru"
) { require_once './lang/ru.php';
} else if ($_SESSION['lang'] == "en") {
require_once './lang/en.php';
}
}
}

Answer 1

Haven't found any ready-made solutions?
In any case, this piece will work better in this form:

if (empty($_SESSION['lang'])) {
  preg_match('/^\w{2}/',$_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches);
  switch (strtolower($matches[0])){
    case "ru": 
      $_SESSION['lang']="ru";
      break;
    case "en": 
    case "uk": 
    case "us": 
    default: 
      $_SESSION['lang']="en";
      break;
  }
}
require_once './lang/' . $_SESSION['lang'] . '.php';