If you've created a SwiftKey Account, then you can remove your account and all data attached to your account in the following way:
- Open SwiftKey
- Tap ‘Account'
- Tap on your account
- Press ‘Delete account’ and confirm
You can also simply log out of your SwiftKey Account, which will stop sending any further data to the SwiftKey servers.
If you haven't created a SwiftKey Account, for instructions on how to remove the personal language data stored on your device please see this article, 'How do I clear data stored on my device?'.