Throne was discussed at 5.34. As there, it may be translated simply as “the place from where God rules.” But, since the idea of sitting is explicit in the verse, a better sentence may be “the seat from where God rules.”
Him who sits upon it is also a reference to God, and in some languages this may need to be made explicit: “God, who sits on it” (Die Bibel im heutigen Deutsch).
Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .