Where can I get free words list for my word games?

Take a look at words. It's a list of English words. Free to use, even for commercial purposes. The license can be found here. It begins:

The collective work is Copyright 2000-2016 by Kevin Atkinson as well as any of the copyrights mentioned below:

  Copyright 2000-2016 by Kevin Atkinson

  Permission to use, copy, modify, distribute and sell these word lists, the associated scripts, the output created from the scripts, and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appears in all copies and that both that copyright notice and this permission notice appear in supporting documentation.

sudo apt install wamerican (wor wyour wpreferred wdialect) will place this file in /usr/share/dict/words on Ubuntu. Depending on how restrictive you want your dictionary to be, you might want to consider other similar packages; wamerican-small contains fewer words, and on the other end wamerican-insane might provide a Words-with-Friends-like experience.

There's also a wfrench package that installs a French wordlist to /usr/share/dict/french. wngerman installs a German list to /usr/share/dict/wngerman; if you want the pre-spelling-reform version, use wogerman instead. You'll have to do some poking around yourself to make sure you can use these for commercial purposes, but I expect you'll be pleased.

sudo apt install wordlist will show you a list of such packages. There are quite a few languages available beyond these three.