fr 
fr-Latn
fr-Latn-FR
fr-Latn-419
fr-FR
fr-y-myext-myext2
apa-Latn # ISO 639 can be 3-letters
apa
apa-CA
i-klingon # grandfathered with singleton
no-bok # grandfathered without singleton
mn-Cyrl-MN
mN-cYrL-Mn
fr-Latn-CA
en-US
fr-Latn-CA
i-enochian # Grand fathered
x-fr-CH 
sr-Latn-CS
es-419
sl-nedis
de-CH-1996
de-Latg-1996
sl-IT-nedis
en-a-bbb-x-a-ccc
de-a-value
en-x-US
az-Arab-x-AZE-derbend
es-Latn-CO-x-private
ab-x-abc-x-abc # anything goes after x
ab-x-abc-a-a # ditto
i-default # grandfathered
i-klingon # grandfathered
en
de-AT
es-419
de-CH-1901
sr-Cyrl
sr-Cyrl-CS
sl-Latn-IT-rozaj
en-US-x-twain
zh-cmn
zh-cmn-Hant
zh-cmn-Hant-HK
zh-gan
zh-yue-Hant-HK
en-Latn-GB-boont-r-extended-sequence-x-private
en-US-boont