Much thought has gone into selecting Scrabble and other dictionaries for this app. The goal is to present reasonably current files that can be freely distributed. There is a tension between being up to date and licensing issues. But since this app is just for fun, the lack of being absolutely current and official should not be a problem.
The up-to-date, official Scrabble dictionary for the USA, Canada, and Thailand is called the NASPA Words List. This is the dictionary that is used in tournaments but it must be licensed and a fee paid. For this reason it is not included in the app.
The NASPA Word List is also called TWL. I was able to find a license-free version of this dictionary and it is included in the app.
Outside of the USA, the Collins Scrabble Words is used for tournaments. It used to be called SOWPODS. I was able to find a reasonably current plain text file of this dictionary. It has the most words of any of the dictionaries. There is one dictionary (COLLINS) with only words and another (COLLINS_DEFINE) with definitions. Currently, only the Collins dictionary has an option for definitions.
The OSPD (Official Scrabble Players Dictionary) is an older version that NASPA was built from and was the official dictionary for tournaments in the USA and Canada. "Offensive" words were removed from this version. It has the least number of words of the other dictionaries.
find-words-java now has a crossword function. For this I thought I would include a standard dictionary to use. I found the words file on my Linux box (/usr/share/dict/words). I removed words with apostrophes as these would not be able to be processed by the app. I also removed words that differed only by a capital letter, so for instance, if the words "Ant" and "ant" were both in the dictionary, only "ant" would remain. I have written a Python script that will perform these tasks for you, called clean_words.py.
Any plain text file that ends in .txt and has one word per line can be used. Put the file in src/main/resources/dicts and add the name of the file without the .txt to src/main/java/net/snortum/scrabblewords/model/DictionaryNames. If your dictionary has definitions, place a tab (\t) between the word and its definition on the same line. (Because of this, definitions cannot have tabs in the them.)
Have fun!