In our WinPE pass we've got this little bundle in for our 64-bit install,
<component name="Microsoft-Windows-International-Core-WinPE" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<InputLocale>0809:00000809</InputLocale>
<SystemLocale>en-GB</SystemLocale>
<UILanguage>en-US</UILanguage>
<UserLocale>en-GB</UserLocale>
</component>
And this for our 32-bit scripted install,
<component name="Microsoft-Windows-International-Core-WinPE" processorArchitecture="x86" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<InputLocale>0809:00000809</InputLocale>
<SystemLocale>en-GB</SystemLocale>
<UILanguage>en-US</UILanguage>
<UserLocale>en-GB</UserLocale>
</component>