Slide to type in finnish

I don't think so. Finnish seems not to be supported (at the moment).

See this support page:--> https://www.apple.com/ios/feature-availability/ and look at the section "QuickType Keyboard".

Click on the sub-sections "Predictive Typing" and/or "QuickPath".

If you'd like, let Apple know directly your request by submitting it here:--> Apple – Product Feedback

Regards

Giulio