If you want to edit how mods do certain stuff the config folder is your friend.
If you want to change the appeariance of the blocks/items in the mod you can create a ressource pack for it.
If you want to translate the mod you can open the mods .jar file with a program like WinRar and search for the .lang file. I think you can find it in the assets folder. Make sure to save the file as a new one formatted as UTF-8 and with the correct name.
In the case you want to create the ressource pack or the translation you can tell the mod author what you created. Maybe he is glad that you share your translation or ressource pack with him and he will include it in his mods description
You can find the lang files of mods in "<Modname>.jar\assets\<ModID>\lang\en_US.lang
If you want to change the message that appears, the so called splash text, in the main menu you need to create a resource pack. You can do that by looking in a tutorial. I placed a resource pack with a simple structure to replace the splash text in my Dropbox for you