Feel free to remove / delete / overwrite the i18n.js that is currently in the folder - it is the same as the en-gb.i18n.js file in the same folder!
What you can do is rename i18n_pol.js to pl-PL.i18n.js (assuming you're using standard Polish) then make a copy of that file and rename the copy to i18n.js, to replace the current one.
1) when you upload the game it only run from the i18n.js file
2) it's still a good idea to save your own work in a few places, including elsewhere on your computer, just in case.
3) it looks like there is a plan to add a language selector to the game, so that pl-PL.i18n.js will be ready to go :)