You are only required to prove to the Citizenship Office that you can manage independently in everyday situations using English. Acceptable evidence is documented on
this page.
Although I cannot be certain, there is also
information that suggests that having previously held a visa that has an English language requirement is considered sufficient proof as far as automated checking is concerned -- but, if English is not your first language, you should still submit acceptable supporting evidence, per current guidance from the Citizenship Office, in case the automated check does not pass.
If the Citizenship Office believes that your English language ability may be questionable, you may need to be interviewed by a Citizenship Case Officer. If the Citizenship Case Officer forms the view that you have insufficient knowledge of the English language,
current policy is that you must be offered the opportunity to attend a further interview in a further 6 months, at which point the final decision will be made.
Finally, if you are eventually assessed as not clearly meeting the English language requirement, it is possible (but difficult) to have this requirement waived under
Section 8(8) of the Citizenship Act. To obtain this waiver, you need to satisfy the Citizenship Office that, because of your age or standard of education, or for any other reason personal to you, you would suffer undue hardship if you are required to have to have sufficient knowledge of the English language.