[SuperZen]

let result = []
for (let index = 0, string = 'Hello, World!Привет, Мир!'; index < string.length; index++) {
  if (result.length === 0 || !/[a-zа-яЁё]/ig.test(string[index])) {
    result.push(string[index])
  } else {
    result[result.length - 1] = result[result.length - 1] + string[index]
  }
}
console.log(result);

const res = [...'Hello, World!Привет, Мир!'].reduce((acc, letter) => {
  !acc.length || !/[a-zа-яЁё]/ig.test(letter)
    ?
    acc.push(letter)
    :
    acc[acc.length - 1] += letter
  return acc
}, []);
console.log(res)