Tests a segment in a url with a country code (en, nl) or locale pattern (en-UK, nl-BE)

resolveLocale.js

function resolveLocale(url, segmentIndex, defaultLanguage = 'en') {
  const segments = url.split('/');
  // look for either ISO 3166-1 alpha-2 or combined with ISO 3166-2
  const pattern = /\b\w{2}(-\w{2})?\b/gim;
  return (segmentIndex < segments.length && pattern.test(segments[segmentIndex]))
    ? segments[segmentIndex]
    : defaultLanguage;
}