Per  https://tc39.github.io/ecma402/#sec-canonicalizelanguagetag
6.2.3 CanonicalizeLanguageTag ( locale )
The CanonicalizeLanguageTag abstract operation returns the canonical and case-regularized form of the locale argument (which must be a String value that is a structurally valid Unicode BCP 47 Locale Identifier as verified by the IsStructurallyValidLanguageTag abstract operation). A conforming implementation shall take the steps specified in the “BCP 47 Language Tag to Unicode BCP 47 Locale Identifier” algorithm, from Unicode Technical Standard #35 LDML § 3.3.1 BCP 47 Language Tag Conversion.

And
http://unicode.org/reports/tr35/#BCP_47_Language_Tag_Conversion
"
2. If the BCP 47 primary language subtag matches the type attribute of a languageAlias element in Supplemental Data, replace the language subtag with the replacement value.
If there are additional subtags in the replacement value, add them to the result, but only if there is no corresponding subtag already in the tag.
"
and
https://www.unicode.org/repos/cldr/trunk/common/supplemental/supplementalMetadata.xml
...
<languageAlias type="aar" replacement="aa" reason="overlong"/>
 <!--  [Afar]  -->
...
<languageAlias type="ces" replacement="cs" reason="overlong"/>
 <!--  [Czech]  -->
...
<languageAlias type="heb" replacement="he" reason="overlong"/>
 <!--  [Hebrew]  -->