If you mean native English:
* King William IV (reigned 1830-1837) is often cited as the only monarch who didn't speak English natively. He was born in Hanover, Germany, and his first language was German. However, he was fluent in English and spoke it publicly.
If you mean any English:
* Mary I (reigned 1553-1558) was born in Spain and raised in the Spanish royal court. While she learned English after ascending to the throne, she was not fluent and preferred to speak Spanish.
It's important to note that while both monarchs were not native English speakers, both learned the language and were able to communicate in English.
Therefore, the answer to your question depends on your definition of "unable to speak English."