Not to be rude but... How are you going to edit the messages if you can't even find the FactionCommand class or know how to use DevTools.
You'll find the FactionsPro repository on GitHub here: https://github.com/poggit-orphanage/FactionsPro. Make an account on GitHub, fork that repo, clone your fork into your local PocketMine server plugins folder, change the name to FactionsProSRC (or anything else so your config files don't finish up in the src folder), make your changes to the code (for example, add a config file to let users translate strings), commit and push changes to your forked repo, then you can use DevTools to create a .phar file and/or submit a pull request to the poggit-orphanage repo.