Hello. You don't find that in the language file because it is not text. It is part of the image. Inside the DT Register package, you'll find the source image for all buttons in all colors. You can take those blank buttons and add your own text to them in an image editor... then save that button with the EXACT same filename as the original image, and replace it here where the original images are located:
/components/com_dtregister/assets/images/