Install correct Language:
For all MSDN & VLSC users out there, if you want to make use of Cortana remember to install US-English version or another supported language of Windows 10. If you do not do that, there is no way to enable Cortana at the moment.
Don’t change default language
Install in with default keyboard, language. Just leave everything to default.
Don’t log in with a Microsoft Account Yet
It seems that if you log in with your MSA/Azure AD account even on a supported language Cortana will be disabled because your Azure AD or MSA might have regional settings.
Choose “Join a domain”
She will prompt you to log in with your Microsoft Account, that’s okay now
Connect your Microsoft Account
Then connect your MSA and log in/out
You can now use Cortana