Add component amd64_Microsoft-Windows-International-Core (replace amd64 with x86 if 32 bit Windows) to settings pass OobeSystem and fill in locales, EN-US in case of US English and region USA.
Originally Posted by UkiwiS
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The region will be set automatically based on locales, for instance if I set all locales to SV-SE (Swedish in Sweden) region will be Sweden, but using SV-FI (Swedish in Finland) region will be Finland.
If using other than default locales of the install media (install.wim), required languages need to be installed before Sysprep. For instance if using US English install media you want locales set as UK English, you need to install UK English language pack in Audit mode before sysprepping, then set all locales to EN-GB.