The Add to Cart message is displayed via Rules in response to the "After adding a product to the cart" event. The rule configuration that does this uses a special action called "Display a translatable Add to Cart message" that does not allow the message itself to be configurable; this is how we are able to enforce translatability.
Fortunately, since it's powered by Rules, you can edit that configuration or just disable it and create a new one that you manage in code to display the message you want using the generic "Show a message on the site" action.
Another module that may interest you is the Commerce Add to Cart Confirmation module that really brings the Add to Cart message front and center via a modal dialogue and Views powered output. This is the module Commerce Kickstart 2.x uses to show the popup after the Add to Cart form is submitted.