i18n: Silent failure when using ISO 639-3 language tags that have ISO 639-1 equivalents

[jmooring]

Reference: https://discourse.gohugo.io/t/35938

This behavior was introduced with v0.76.0 where we upgraded nicksnyder/go-i18n from v1 to v2.

Language Tag	Description	i18n
eng	ISO 639-3	❌
nld	ISO 639-3	❌
es	ISO 639-1	✔️
zzz	artificial	✔️
mylang	artificial	✔️

MRE

git clone --single-branch -b hugo-forum-topic-35938 https://github.com/jmooring/hugo-testing hugo-forum-topic-35938
cd hugo-forum-topic-35938
hugo server

Notes

1. I suspect this is a limitation of nicksnyder/go-i18n, but do not know for certain. For example, the CLDR data has entries for en and nl, but not for eng and nld.
2. Using ISO 639-3 also prevents localization of dates, currency, numbers, and percentages. That is a related but separate issue, Use Language.LanguageCode as localization key, falling back to language.Lang #9109.

[github-actions]

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