Don here from Shopify. :)
I just wanted to double check with you how you have been trying to edit language in your theme?
If you haven't already, you can access the language editor via Online store > Themes > Actions > Edit languages.
You will see there some default divisions among areas of the theme, which includes 'Checkout & system'.
You should be able to simply enter the language you wish to update into the relevant field and save your changes as shown here:
If, for any reason, you're unable to edit this language then please let us know so we can check into this for you.
It could conceivably be the case that some third-party themes may not allow you to edit this, though that would be pretty unusual in my experience.
If you would like us to look into this for you, you can just email firstname.lastname@example.org and then quote the ticket number here in this thread so I can access and discuss your account securely. :)
All the best!