That really depends on the type of game. RPGMaker and WolfRPG games can be decrypted and edited directly with the proper editor, like using RPGMaker VX Ace for games made with it. You will need a 3rd party decrypter software, usually called 'RPG Maker Decrypter', and you should be able to find it on Google pretty easily.
If it's RPGMaker before MV, the other option is to use a program called RPGMakerTrans. You'll still need to decrypt it, but this will extract the text into txt files that can be edited, and then turned into a patch. This way, you don't have to muck around the game files searching for the text, as it's easy to miss some, or mess up the game code in some way.
Searching Google for these things should bring you to appropriate sources. I'd list some sites, but IDK if they are allowed to be mentioned here.
Also, note that this is just for unofficial fan translations. If the dev asks you to stop, then you have to.