Is being able to speak English a reasonable qualification for the job

Yes, for some jobs this would be a reasonable qualification. If the job involves reading documents in English or having a front facing position with people who only speak English then this would be reasonable. For example, if you lived in Tokyo but you were working at a hotel that had mostly English speaking guests, it would make sense to have someone who could converse with them in English.

English is necessary to perform the job effectively for communications with customers, coworkers, or supervisors who only speak English in emergencies or other situations in which workers must speak a common language to promote safety.