Issues with `i18n_lang` field in Sanity document internationalization plugin

{
  "idStructure": "subpath",
  "referenceBehavior": "weak",
  "base": "en",
  "languages": ["en", "fr"],
  "fieldNames": {
    "lang": "i18n_lang",
    "references": "i18n_refs"
  }
}

export default {
  type: 'document',
  name: '...',
  title: '...',
  i18n: {
    base: 'en_US',
    languages: ['en_US', 'nl_NL'],
    fieldNames: {
      lang: '__i18n_lang',
      references: '__i18n_refs',
      baseReference: '__i18n_base',
    },
  },
  fields: [...],
}

  initialValue: {
    __i18n_lang: 'en_US',
  },

import client from "part:@sanity/base/client"
import cq from 'concurrent-queue'

// Create a queue to limit the rate at which you write changes to Sanity
let queue = cq().limit({ concurrency: 2 }).process(task => {
  return new Promise(function (resolve, reject) {
      setTimeout(resolve.bind(undefined, task), 1000)
  })
})

const studioClient = client.withConfig({apiVersion: '2021-03-25'})

const mutateDocs = async () => {
  //Fetch the documents you need to mutate
  const query = `*[_type == '<your-type>' && !defined('i18n_lang')]`
  const docs = await studioClient.fetch(query)
  // Loop through all of the docs returned from our query
  for (const doc of docs){
    queue(doc).then(async () => {
    // Add a message to help us know the upload is happening
      console.log(`Mutating ${doc._id}`)
      // Tell the client to patch the current document
      studioClient.patch(doc._id)
        // Set the field
        .set({
          i18n_lang: 'en'
        })
        // Commit the changes
        .commit()
        .then(updatedDoc => console.log(`Hurray, the doc is updated! New document:${updatedDoc._id}`))
        .catch(err => console.error('Oh no, the update failed: ', err.message))
    })
  }
}

mutateDocs()

// execute this script by running 
// $ sanity exec ./path/to/mutateDocs.js --withUserToken