Word References

You can play Kiitos completely offline with a dictionary for challenges and use that as your agreed reference. Depending on which dictionary you use the list of acceptable words will be different.

There is another word game that has a bit of a following and is played at tournaments around the world. Some people like it. That game has an official word list called the Collins Scrabble Words, which was formerly known as SOWPODS. For all intents and purposes this pretty much suffices for a Kiitos reference. Of course 2 and 3 letter words aren’t used in Kiitos, and the word list tends to top out at 15 letters.

There are many online tools for looking up valid words. They vary as to how up to date they are in terms of words against the official tournament words, but in practical terms that doesn’t matter. Here’s a couple:
https://www.wordplays.com/scrabble-dictionary
https://www.collinsdictionary.com/scrabble/scrabble-word-finder/

There are also many word checker apps you can install under Android or IOS.
Personally I use the Royal College Scrabble Club app on my Android phone, since it is free. Visit the Google Play or Apple equivalent stores on your device.

Is it OK to include word lists from other sources?
By mutual agreement before you start playing you can use any source of words you like. For instance if you want to include the names of Pokemon as acceptable words. then sure, you may do so.