UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.40 Safari/537.36

Steps to reproduce the problem:
1. Go to any page with text box, where you can write text, e.g: paste.debian.net
2. Write something in Finnish, for example "Hei, mitä sinulle kuuluu?" (in English: "Hey, how are you?")
3. All four words are not spelled or proofed. 
4. There is no way to turn on spell-checking for Finnish (or for Saami, 

What is the expected behavior?
Finnish spell checking should be enabled and should show misspelled words.

What went wrong?
Google Chrome expects and supports Hunspell dictionaries.

Finnish language is inflected (read Wikipedia to understand what it means), it cannot be properly spell checked OR proofed using static dictionaries (like Hunspell wordlist).

https://en.wikipedia.org/wiki/Finnish_grammar

This is why the Free Linguistic analysis / inflection software for Finnish language, titled "Voikko" was developed:

http://voikko.puimula.org/

Voikko can be included as a library or accessed via the Enchant spellchecking library:

https://github.com/AbiWord/enchant

Voikko also support other inflected languages, like Saami and potentially Icelandic, which again, cannot be spell-checked using static Hunspell dictionaries.

Did this work before? No 

Chrome version: 63.0.3239.40  Channel: beta
OS Version: 10.0
Flash Version: Shockwave Flash 27.0 r0

You can get a badly working kludge, i.e. a static dictionary for Finnish, that 'sort of works some of the time' from the Debian myspell-fi package, that contains a Finnish static word list.

https://packages.debian.org/wheezy/myspell-fi

However, as stated, Finnish language is inflected and as such, any such static dictionary always falls short.

Yet, it would be better than nothing to even get that level of proofing support in Google Chrome.

Real support for languages with inflection (using an existing API like Enchant) or using OS level specific proofing (Windows 10 has system level proofing, which Edge browser uses). This system level proofing supports languages like Finnnish.