In most countries, yes, you need a teaching certificate to teach at the high school level, even if you're teaching English. The specific requirements vary widely by country and sometimes even by state/province within a country. You'll need to research the requirements of the specific location where you intend to teach.
Some countries might have exceptions for specific programs or situations (e.g., a short-term teaching position at a private school), but generally, a teaching certificate demonstrating competency and training in pedagogy is expected.