Which British monarch was the last to not be a native English speaker?

King George II of Great Britain and Ireland (1683-1760) reigned from 1727 to 1760 (his death). Although raised in Germany, his first language was French until he learned German at the age of four.

https://wiki2.org/en/George_II_of_Great_Britain+Milds.2

King George II was indeed fluent in French and German before he learned English as a third language (and Italian as a fourth).

However, his father and predecessor as a King of Great Britain and Ireland didn't even bother learning English. When he was crowned on 20th October 1714, the ceremonies had to be conducted mostly in Latin as he could not speak English and his ministers could not speak German.

https://www.westminster-abbey.org/about-the-abbey/history/coronations-at-the-abbey/coronation-stories