diff --git a/icu4c/source/common/unicode/locid.h b/icu4c/source/common/unicode/locid.h
index a6fbbb70418..36575f13f2c 100644
--- a/icu4c/source/common/unicode/locid.h
+++ b/icu4c/source/common/unicode/locid.h
@@ -984,7 +984,10 @@ public:
     static const char* const* U_EXPORT2 getISOCountries();
 
     /**
-     * Gets a list of all available language codes defined in ISO 639.  This is a pointer
+     * Returns a list of all unique language codes defined in ISO 639.
+     * They can be 2 or 3 letter codes, as defined by
+     * <a href="https://www.ietf.org/rfc/bcp/bcp47.html#section-2.2.1">
+     * BCP 47, section 2.2.1</a>. This is a pointer
      * to an array of pointers to arrays of char.  All of these pointers are owned
      * by ICU-- do not delete them, and do not write through them.  The array is
      * terminated with a null pointer.
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/util/ULocale.java b/icu4j/main/classes/core/src/com/ibm/icu/util/ULocale.java
index 89d1baa452a..0f3591c98b9 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/util/ULocale.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/util/ULocale.java
@@ -877,8 +877,10 @@ public final class ULocale implements Serializable, Comparable<ULocale> {
     }
 
     /**
-     * Returns a list of all 2-letter language codes defined in ISO 639.
-     * Can be used to create Locales.
+     * Returns a list of all unique language codes defined in ISO 639.
+     * They can be 2 or 3 letter codes, as defined by
+     * <a href="https://www.ietf.org/rfc/bcp/bcp47.html#section-2.2.1">
+     * BCP 47, section 2.2.1</a>. Can be used to create Locales.
      * [NOTE:  ISO 639 is not a stable standard-- some languages' codes have changed.
      * The list this function returns includes both the new and the old codes for the
      * languages whose codes have changed.]