"We want rice."
Translation:Dyn ni eisiau reis.
No, just an acceptable variant.
dan ni reflects how dyn ni is pronounced in parts of north Wales, and it is quite common for people who pronounce it like that to spell it that way when writing informally. It is also taught on some courses in that area, which is why we accept it, and a very few similar examples, on this course.
Stick with using dyn ni unless you have a strong reason not to. Be aware of dan ni as a variant.