1. You need split your array into chunks (2 element per chank), for example by indexing like this:

const resultIndex = Math.floor(index / 2)

index here is ininitial array index, resultIndex - new index of current element (chunk index actually).

2. Join them as you wish using .join():

result.map(chunk => chunk.join('+'))

Full code:

const arr = [ 'CP-en', 'AR-es', 'CP-en', 'BE-fr', 'CP-en', 'CZ-cs' ]
const result = []

for (let index in arr) {
  const resultIndex = Math.floor(index / 2)
  if (!result[resultIndex]) result[resultIndex] = []
  result[resultIndex].push(arr[index])
}

console.log(result.map(chunk => chunk.join('+')))