ICAO
International Civil Aviation Organization.
The International Civil Aviation Organization (ICAO) is a United Nations specialized agency that establishes international standards and recommended practices for civil aviation. ICAO Annex 1 (Personnel Licensing) sets the language proficiency requirements for pilots and air traffic controllers operating on international routes.
ICAO requires that pilots and ATCs operating in international airspace demonstrate at least Level 4 (Operational) proficiency in either English or the language used by the station on the ground. ICAO publishes Doc 9835 (Manual on the Implementation of ICAO Language Proficiency Requirements) which defines the six proficiency levels and the six assessment criteria.
ICAO certifications are administered by national aviation authorities (the FAA in the US, EASA in Europe, etc.) following ICAO standards. Most national authorities accept ICAO-compliant assessments from credentialed testing organizations.