The locales folder is for translation – it tells the game how to translate the mod’s written contents (such as names and tooltips) into various languages. How it works is that each .json in that folder contains a copy of the mod’s written text in each language that a translation exists for, so you’d have, say, fr.json for a French translation or something like that.
You need to at least have one language AFAIK, you only need files for other locales (i.e. localisations) if you’re supporting those languages. Of course you can talk to some of the awesome community translators if you want to have a crack at that; but it’s not essential to get your mod working – just use the existing mod as an example again.
The biggest thing to be aware of is to make sure all your “references” match up. Ctrl+F is your friend here; since it helps you track down anything you’re looking for. When I mod other games I like to use it to check through all the new stuff I’ve added, it’s a little more “targeted” than scrolling through the entire page hahah. You can even use the find-and-replace tool to speed things up further.