Hey there, when using Projectiles-1.0.0-1.20.1-Multi.jar, a bunch of Minecraft localisation strings are broken.
In the console, instead of "Preparing spawn area: xx%", I just get menu.preparingSpawn. Instead of chat messages showing, it comes up as player.chat or something along those lines. Even the discord integration bot shows advancements and death messages as advancement advancements.story.mine_diamond.title, advancements.story.mine_diamond.description
I'm on Fabric 1.20.1 if that helps.

When removing the mod does it fix anything? and without a debug.log, / crash report / mod list/ arch version I cant pinpoint anything. but from what i know projectiles doesn't touch any localization code.

It's because you replace the en_us.json with your own, that only includes a single entry, killing every other localization. Remove the file and it's fixed.