LONDON — Kate, the Princess of Wales, announced Monday that she has completed her chemotherapy treatment and will undertake a light schedule of engagements until the end of the year.

“Doing what I can to stay cancer free is now my focus,” she said in a video message, some six months after revealing she had an unspecified form of cancer following intense public speculation about her health.

“The last nine months have been incredibly tough for us as a family,” Kate said, adding that…