BigCommerce does not currently have a way to modify the language variables / messages in the templates. The only workaround for this is to make the changes directly in the template files either by replacing the variable or using jQuery to directly modify the HTML element.
For example, if a country is not covered by a Shipping Zone, when a customer selects that specific Country, there will be a message shown: "Unfortunately we can't ship to your location. Please select another location and try again." This post will show you how to change that text.
Just place the following code into the footer.html template file:
You can apply this same method to change the text on other variables as well (not just the shipping error). Pretty much every text that is displayed on the page can be changed. If you are stuck, or need help applying this script to your BigCommerce template then just contact us via email or post a comment down below.