I lost my notes. The post explains a lot that is not considered while "only a translation". From the other specific translations: dates, expressions, name format, etc...

to the undergoing system to be maintained: a N+1 feature (each changes needs to be internationalized and localized accordingly), testing, user tests needs to be in other languages too.

DateTimeFormat, NumberFormat, ListFormat, RelativeTimeFormat (!), PluralRules, DisplayNames

The language tag registry exists on https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

The document provides decisions to handle i18n naming conventions.

See https://pontoon.mozilla.org/