You do not have to change the label via JS. You can find the label text by navigating to
then find OrderResources
from the left panel.
Then in the search field type "add to cart", edit both fields with the desired text.
If you have any other problem, feel free to contact us again.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items