Windows – Install language pack via command line

Without actually trying it myself, I cannot be sure. However, I am assuming that you are trying to get a tablet with Windows RT on it.

To learn how to get additional language packs, see Languages in Windows 8 and Windows RT, where it shows how to do it on the desktop, as well as in the Modern UI interface (quoted below):

Here's how to do it:

1. Open the Search charm, enter language, choose Settings, and then tap or click Add a language.
   • In the Language Control Panel, tap or click Add a language.
2. Browse or use the search box to find the languages you want to add, and then double-tap or double-click the language you want to add.

And here is what AndrewTechHelp in Configuring Windows 8 and Windows RT To Spell Correctly Outside The US says:

Microsoft has made language packs available to ALL editions of Windows 8 and also Windows RT

And even Office in Building Office for Windows RT says:

One area that was identified for space savings was language packs. Many OEMs load multiple languages onto the systems they ship... ...If a user decides later that they want one of the languages that were removed the system will automatically download the required files from Windows Update.

That suggests that you can get language packs even in Windows RT.

Download the EN-GB MUI you need:

32Bit: http://fg.v4.download.windowsupdate.com/d/msdownload/update/software/updt/2013/09/lp_43e709fc9932e26431ff87e163daaaeae15677eb.cab

64Bit: http://fg.v4.download.windowsupdate.com/d/msdownload/update/software/updt/2013/09/lp_55c5ff4da1d83ae6f4ebd1f8c020cb80ceda7da8.cab

Rename the file to LP.mlc and double click it to install it.