ja
ban
pt-BR
hy-Latn-IT-arevela
en-GB
zh-Hant
sgn-BE-NL
es-419
en-GB-boont-r-extended-sequence-x-private

# grandfathered
zh-hakka
i-default
i-navajo

# https://github.com/sebinsua/ietf-language-tag-regex/blob/master/test.js
de-CH-1901
qaa-Qaaa-QM-x-southern

# private-use subtags are case-insensitive (RFC 5646 §2.1.1)
X-fr-CH
de-X-foo

# multiple distinct (non-duplicate) variant/extension singleton subtags remain valid
sl-rozaj-biske
en-u-co-phonebk-t-en-US