As a supplement to this SE question; "What are the requirements to become a language proficiency examiner?" (something I never heard of...) What are ICAO English language requirements?
ICAO defined a framework of language proficiency levels for aviation. It ranges from level 1 ("Pre-elementary") to level 6 ("Expert" / native speaker). Level 4, "operational", is the minimum required for flight crews. Many airlines require at least level 5 from their pilots.
Native speakers can get level 6 with a simple proof of that they're a native speaker (I heard recently that e.g. for a German license the authorities see it as sufficient if the person visited an English speaking school). Non-native speakers do assessments to certify their language proficiency. An assessment for level 4 has to be repeated every 3 years, level 5 lasts 6 years and level 6 is life-long.
For languages other than English, there are assessments / language proficiency certifications that are regulated by country-specific authorities and it's practice in at least some countries that native speakers can get their native language put into the license as level 6.