you must either provide an evidence of your English language skills when you apply for this visa or show that you hold a valid passport from UK, USA, Canada, New Zealand or Republic of Ireland.
Generally the evidence is an English language test result. e.g. IELTS, PTE, TOEFL etc. The test results must not be more than 3 years old and you must have scored an overall score of 6.0 with a minimum score of 5.0 in each component in IELTS (or equivalent).