Using this extension (there is only one Vqmod file and no any files will changed), you can change the default label of the Add to cart button on its own and even separate for each product. If the product label is not specified, the default Opencart caption uses.
Operation of this module was tested with Opencart versions 1.5.6.x, but I believe it will work with later versions. If you find any error or bug, please inform us about and we will fix it shortly.
Localizations: English, Russian (If you need another, just write us).
Support email: firstname.lastname@example.org