I don’t know if this the correct way of doing this so please make sure you backup your EE system, website, and database as this might cause problems and I’m not sure if it will work correctly. The file you want to change is located here:
/system/expressionengine/language/english/core_lang.php
If you are going to change the language I would recommend making a duplicate copy of the /system/expressionengine/language/english/ folder and call it “french” for example. Then open “/system/expressionengine/language/french/core_lang.php” file or what ever language you called it and apply the translation changes you want to make to it.
Once you are done applying your translation changes go to the CP under “Admin > General Configuration” and set the field for “Default Language” to the language you called the folder from above.
Now I think this will change the values you want to the language of your preference. I have not tested any of this so please be careful when doing this so you don’t break your CP and website. I would recommend trying this on a dev/staging EE installation in case anything goes wrong. I hope this helps.
Mike