This requirement is separate from every academic exam in the application, and it is the one students most often discover late, when there is little time left to sit a test.
Which test is accepted depends on the institution
TOEFL and IELTS are accepted almost everywhere. The Duolingo English Test is accepted by many institutions and not by all, and the list changes between cycles.
The reliable move is to check the specific institution's page for the current year rather than to rely on a general list, because a test accepted last year may not be this year.
The differences between the tests
TOEFL is delivered on a computer and is closer in style to academic listening and reading. IELTS has a face to face speaking component in its usual format. The Duolingo test is shorter, cheaper and taken at home, which is what makes it attractive and also what makes some institutions cautious.
The choice is usually settled by which test the target institutions accept rather than by which suits the student, since the first constraint is narrower.
Exemptions
Many institutions exempt students who have completed several years of secondary education in English. The number of years and the required documentation differ.
Where an exemption applies, it saves an exam and a fee. It is worth checking before booking a test, and the answer is on the institution's page.
The SAT does not substitute
A strong SAT Reading and Writing score is not an English proficiency certificate. The two exams measure different things and are read separately.
A student in Turkey with a high SAT score may still be required to sit TOEFL or IELTS, and this surprises applicants every year.
Score validity
These certificates expire, usually after two years. A test taken early in secondary school can be out of date by the time the application is made.
This is one requirement where taking the exam early is not an advantage.
Where it sits in the plan
After the academic exams and before the application deadline, with the validity period checked against the intended start date. Because it is short to prepare for relative to SAT or IB, it is easy to schedule, and easy to forget entirely.