You'll need to buy the "Windows Anytime Upgrade" to Ultimate for language packs. Go to the Start Menu, type "Windows Anytime Upgrade", w/o the quotes. Pricing depends upon where you live.
You'll be emailed a key after purchase, in the US, it's over $100. It's a 10 minute upgrade, with no loss of files, no format, everything will be the same. All that you're doing is buying a key to "unlock" what you already have. Make sure that you copy that key & store it in a safe place, as if you need to re-install your OS, you'll need to re-enter the key to upgrade again.
Once you've upgraded, the English language pack is an update that you'll be able to select from.
Hope that my post explains your options.
Best of Luck,
Cat