Be extremely wary of "free standalone language pack download" sites. Many contain malware, keyloggers, or adware. Legitimate MUI packs come as large .ISO files or a suite of .MSI installers.
This is the most difficult part of the process. Microsoft officially ended support for Windows XP in 2014, and the download links on Microsoft.com are long dead. You cannot download them via Windows Update anymore. windows xp sp3 language pack