Hey @Liam-Shopify Sorry to @ you on here but we have no other way to communicate now.
Did Shopify recently add some form of checking to the locales JSON files during updates?
We have seen a lot of merchants over the past few weeks unable to update themes due to some invalid JSON in older theme versions, even when this is rectified manually the update still fails with JSON errors.
Big problem as merchants can not update some themes right now.