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Merging #1892 (ae32fd5) into master (f055c11) will not change coverage.
The diff coverage is 100.00%.

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I understand the desire to stick to regex based validation, but ISO will never update this spec with breaking changes, as far as I understand it. This specific version of ISO 639 is not likely to be updated ever again. It was last updated in 2002 and has since been reviewed three times without changes.

I cleaned up the list of language codes so it's easier on the eyes, like the list in the ISO3166 validator

I suppose a more complete implementation of this could be isISO639(str, versions) where you can specify which versions of ISO639 you wish to validate against (perhaps just 1 through 5, since version 6 was withdrawn).

The default behaviour without any options specified should match any of the 5 official and current versions of the standard.

Here's how I would implement that:

// declare 5 different sets with all the language codes in them

const sets = [isISO6391Set, isISO6392Set, isISO6393Set, isISO6394Set, isISO6395Set];

export default function isISO639(str, versions = [1, 2, 3, 4, 5]) {
  assertString(str);

  return versions.reduce((result, version) => {
    const currentSet = sets[version - 1];
    return currentSet.has(str) || result;
  }, false);
}

That would be a more complete implementation, but I'm not sure how often that would be used. Especially with the large number of supported languages for ISO 639-3, it might be too much for this project.
Also, I am not sure if the default for matching any of the 5 would be smart for false positives. But I would like to hear the opinions of @profnandaa and @ezkemboi about this

I came looking for this feature today, any updates regarding this functionality?

I agree with the option to specify the version via an options parameter, however, I would set the default version to 1. For now the default set in commit works for me.