The Finnish title should be the title used by the first official Finnish broadcaster. If the series hasn't been broadcasted in Finland, the Finnish title should remain empty.

So, the question is, with which title was this series officially broadcasted in Finland ?

No idea and as far as I know there is no public database that provides this information. With that being said, in all old and current listings for multiple local channels and streaming services (several years old and also for different broadcasters) I could find (without manually digging through every possible source that might have some info), Castle is Castle like it should be. In any case it would be misleading to translate this as "Linna" (even if that is a Finnish name) because it would have absolutely nothing to do with the show. Sure, if most of the results would have been translated, it would be logical but since they were not, it isn't (but it would still be wrong and I'd like to give some feedback to the original translator). I've never seen this show as "Linna" anywhere else but here (and on Trakt.tv where I noticed the name). I left the English name on the Finnish section because a clear name is different from an empty form, if someone happens to simply check it and thinks that the local name is missing (and I can't lock the fields).

@olavinto said:

I left the English name on the Finnish section because a clear name is different from an empty form, if someone happens to simply check it and thinks that the local name is missing (and I can't lock the fields).

If the translated title is the same as the original title, the field should be left blank.

I can't lock the field as I have no proof that it was originally broadcasted in Finland with that title.

A tv guide service that has some programming as far as 2011 with the name Castle (channel was 4, Nelonen): https://www.telsu.fi/castle/-35 (the oldest page)

Also listed in the "Ruutu" following streaming service (Ruutu is operated by the same company that owns the "Nelonen" channel so I guess they still hold local broadcasting rights): https://www.ruutu.fi/ohjelmat/castle

Viaplay is a nordic streaming service from Viasat and they also list the show as Castle: https://viaplay.fi/sarjat/castle

These are all in Finnish though so they might be a little hard to read...

I have locked the field.