However, you might also see Rwsïaidd used, which is more literally translated as "Russian (person)."
Here are some examples:
* Mae hi'n siarad Rwsieg. (She speaks Russian.)
* Rwy'n hoffi bwyd Rwseg. (I like Russian food.)
* Mae'n dod o Rwsia. (He comes from Russia.)
The choice between Rwsieg and Rwsïaidd can depend on the context and the nuance you're trying to convey.