Windows 8 Speech Recognition Language
Solution 1:
This probably has not changed from Windows 7, where the speech recognition language is tied to the language specified for the OS. Try getting and installing the English-US language pack and it should become available as an option for speech rec. Here is a blog that discusses installing language packs in Windows 8.
Solution 2:
Finally found the solution:
1) Download the x64 Language Pack DVD ISO
2) Navigate to the EN-US folder and copy the lp.cab file to a local hard drive
3) Rename the .cab file to .mlc and double click it
4) Run through the wizard to install the language
5) Open Advanced Speech Options and EN-US is now available