更新到最新版本后，Joomla Constant _QQ_已经定义

I am new in Joomla. I have recently update my version from joomla 2.5 to joomla 3.8.11.

I had replace all files of jomlaa 3.8.11 to root directories.

When I access admin side, It gives me error :

Notice: Constant QQ already defined in C:\xampp\htdocs[main_folder]\libraries\src\Language\Language.php on line 18

Anyone master here who can fix it ?