There's no one accepted answer to the question, "Who was the first English king to speak English?" Instead, there are a couple of good possibilities. During the Anglo-Saxon period, different regions of present-day England were ruled by kings who spoke Old English. Offa, who ruled from 757 to 796, was the first of these rulers to be called “Rex Anglorum,” or “King of the English,” so you could argue that he was the first English king to speak English.
After the Norman Conquest
The Norman Conquest of 1066 was a linguistic sea change for England, as Anglo-Saxon rule gave way to a series of kings who spoke a dialect of Old French. These kings had varying degrees of English language ability. For instance, one chronicle suggests that Henry II, who ruled from 1154 to 1189, seems to have understood Middle English but not spoken it. But by the 14th century, English kings were likely bilingual. Scholars Michael Prestwich and Marc Morris agree that Edward I, who ruled from 1272 to 1307, learned English as a child from his tutors. In 1362, the Statute of Pleading made English the official language of Parliament, meaning that lawmakers, including the king, spoke English well enough to conduct legal proceedings in it. Henry IV, whose reign inaugurated the 15th century, was the first English king to speak English as his first language, making him another good answer to the question.