Yet another problem with the Windows 8.1 Preview update.
The iso images files are already publicly available here (which should solve all of this language hurdles) but, here’s a possible solution that Marco Enxuto found if you still want to install it from the Windows Store:
“1. Change your Language and Region to EN-US through Control Panel – Language
2. Ensure you have installed the EN-US language pack.
3. Open registry editor and navigate to HKLM\System\CurrentControlSet\Control\NIs\Language. Find “InstallLanguage” and change this to 0409 (En-US)
4. Save and reboot.
5. Download and install the preview installer from http://windows.microsoft.com/en-us/windows-8/download-preview
6. On reboot, check the reg key above is still set to 0409, then run WSReset.exe in an elevated command prompt. Let this complete, then you should be able to download the 8.1 preview update!”