There is an ongoing push for Air Traffic Controllers (ATCs) to learn English beyond the recommended ICAO Level 4 standard, and instead learn adequate English that allows for a more broader range of communications.
The reasoning behind this is that ATCs are often required to respond to unusual situations that can consist of a wide variety of English constructs. Therefore, the ATCs from non-native English speaking backgrounds may need to prepare and sit for a more advanced level of English compared.
The articles below, although from 1997, describes the reasoning for this push. Please read this article and post your comments or thoughts below.