Firstly, you have to go to your Online Store > Themes > Edit languages.
Then choose language you want to translate:
Follow below steps for all your target languages:
If you see your target language in the list, then no action needed.
Otherwise, press on Other languages… and select your target language.
Go to Checkout & system tab and privide your custome transtation for selected language.
Finally, choose your original language back.
That’s it. Please visit your website, refresh the page and the shopify checkout page will be also translated.
Your Shopify store should be fully translated now.