Turn off automatic language detection - Prestashop

Tools.php

<?php

class Tools extends ToolsCore
{
  /**
  * Change language in cookie while clicking on a flag
  *
  * @return string iso code
  */
  public static function setCookieLanguage($cookie = null)
  {
    if (!$cookie)
      $cookie = Context::getContext()->cookie;
    /* If language does not exist or is disabled, erase it */
    if ($cookie->id_lang)
    {
      $lang = new Language((int)$cookie->id_lang);
      if (!Validate::isLoadedObject($lang) || !$lang->active || !$lang->isAssociatedToShop())
        $cookie->id_lang = null;
    }

    /* If language file not present, you must use default language file */
    if (!$cookie->id_lang || !Validate::isUnsignedId($cookie->id_lang))
      $cookie->id_lang = (int)Configuration::get('PS_LANG_DEFAULT');

    $iso = Language::getIsoById((int)$cookie->id_lang);
    @include_once(_PS_THEME_DIR_.'lang/'.$iso.'.php');
    
    return $iso;
  }
}