Right-click the button and select Command Prompt (Admin) .
For offline deployment on many PCs, integrate language packs into a custom install.wim using DISM: windows 8.1 language pack offline install
Download the required language pack ISO from the Microsoft Volume Licensing Service Center (VLSC) or MSDN. If you do not have access, you can find trusted repositories hosting the .cab files (e.g., en-GB\lp.cab or fr-FR\lp.cab ). Note: Avoid sites wrapping these in .exe installers as they may contain adware. Right-click the button and select Command Prompt (Admin)
This is the safest and simplest offline method. windows 8.1 language pack offline install