capitalize.js

function capitalize(locale, string) {
  let segmenter = new Intl.Segmenter(locale, { granularity: "grapheme" })
  let result = ""
  
  for (let item of segmenter.segment(string)) {
    if (item.index === 0) {
      result += item.segment.toLocaleUpperCase(locale)
    } else {
      result += item.segment
    }
  }
  
  return result
}

console.log(capitalize("en-US", "ééé"))