Hello. The text on the buttons is not actual text... it is part of the image. So you need to replace the actual image. Inside of the DT Register package, there is a zip file containing the buttons in each size and color WITHOUT any text. So you can take the original button, add your own text, save and replace the existing buttons that are found in:
/components/com_dtregister/assets/images/
Or you can just make your own buttons from scratch.