If you still have valid and sufficient proof of English language
proficiency, such as an IELTS score, you can use it again for the MCNB
application (even if 'officially' expired). Else, you can submit your
master’s degree certificate (i.e. diploma) as proof of English language
proficiency. Please upload the MSc certificate in the section where you have to upload the language certificate - make sure to highlight where it is said on the certificate that the languagte of instruction was English!

If you have not finished the English-taught master's degree,
you need additional proof of English proficiency on C1 level.