Native speakers of English will naturally be above the minimum standard of English for this course. Non-native speakers should have an English level of C1 (Advanced) as a minimum. If you are not sure what level your English is there are several websites where you can test it yourself, for example:
C1 is the level represented by the following public exams:
The Pearson Test of English (PTE) General Level 4
Trinity ISE III Trinity GESE Grades 10 and 11
Cambridge CAE Grade A, B or C
IELTS score 6.5-7.5
Michigan ECPE TOEFL score 95-120
TOIEIC Listening: 490-495 Reading 455 Speaking: 180 Writing: 180
Read more: What Level Should My English Be For a TEFL Course?