How to do checkout customisation with new checkout

[Paul_vd_Dool]

I've got a client (Shopify Plus merchant) that wants a warning beneath the address fields (and potentially other fields) in the checkout when the customer fills in anything that is a non-latin character.

Something like this:

I know how to do this with the old checkout where you have the checkout.liquid file to customize. Then you could add a script that does the language checking.

But the customer uses the new checkout and I'm a bit at a loss of how to help them now.

I get that you can customize the checkout further with apps but I'm not sure if I should be looking at Checkout UI Extensions or something else.

A push in the right direction would be greatly appreciated.