Apple bases the region for the user on the billing address as set for the Apple ID account. That in turn set the validity of payment methods: generally you would need a card issued in the same country as the billing address.
Subscriptions are specific to countries. Users can’t change regions while having active subscriptions. Users would need to re-subscribe in a new country. More details in: Change your Apple ID country or region - Apple Support
Siri for Apple TV has a separate rollout compared to other devices. Siri for Apple TV is available in currently 13 countries, and only in the main languages of those countries. Use Siri on your Apple TV - Apple Support
You can give feedback to ask Apple for wider support. Feedback - Apple TV - Apple