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.

There are detailed requirements for each level (e.g. pronunciation, vocabulary etc.), they are usually presented in a big table: