A "nutshell" example, using clak in 4 steps,

// 1. memoize the csv file,
// 2. analyze and persist the precedence, order and position of each locale in at csv file,
// 3. define lazy-lookup functions with default values for each key,
// 4. resolve an i18n value, calling the lazy-lookup function with a locale

const c = clak(csv) // memoize
const langs = c(['en-US','ja-JP']) // analyze and persist column position
const access_denied = c('access_denied', 'no access') // lazy-lookup for key
clak(access_denied, langs, ['ja-JP']) // 'あなたが入れない駄目です'

A longer example with details for each step,

// Supports both two and three letter language id ex, 'en' or 'eng'
const csv = `
"id","key","en-US","ja-JP"
1,"forbidden","you are forbidden","あなたが駄目です"
2,"upstream_error","upstream error","それが駄目です"
3,"access_denied","access denied","あなたが入れない駄目です"
`.slice(1, -1)

// memoize csv to a function returning lang store and tuples
const c = clak(csv, 'en-US') // optional second param, default lang

// lang store defines lang priority; number is lang col position
// ex, [['en-US','ja-JP'], {'en-US': 2, 'ja-JP': 3}]
const langs = c(['en-US','ja-JP'])

// row tuple, if default key has no value from csv, uses scripted default
// ex, ['access_denied','access denied','あなたが入れない駄目です']
const access_denied = c('access_denied', 'no access')

// from each tuple to return the final language-specific value needed.
clak(access_denied, langs, ['ja-JP']) // 'あなたが入れない駄目です'
clak(access_denied, langs, ['en-US']) // 'access denied'
clak(access_denied, langs, ['es-ES']) // 'access denied'

// Where lists of languages are used, a priority order is given so if
// a value for the language at the front of the list is not found,
// a value for the next language in the list will be used and so on.
//
// Using one-item lists and no fallback is fine. Clak will fallback to
// the found or scripted value of the default language, usually "en-US"

clak optionally prints missing row details.

c('forbidden', 'no access') // [!!!] clak: missing row: forbidden
c.warn_disable() // disable warning messages
c('forbidden', 'no access')

clak only returns language strings, so minimal examples of related useful things are demonstrated below..

const tpl = 'Missing fields: {fields}'
const obj = {
  // node and browser native international list-formatting
  fields: new Intl.ListFormat('en', {
    style: 'short',
    type: 'disjunction'
  }).format(['username', 'password'])
}
const msg = Object.keys(obj)
  .reduce((prev, key) => prev.replace(`{${k}}`, obj[k]), tpl)
// 'Missing fields: username and password'

// https://www.w3.org/International/questions/qa-accept-lang-locales
//
// an accept-langauge header might look like this and could be parsed many ways,
//  'en-GB,en-US;q=0.9,fr-CA;q=0.7,en;q=0.8'
const acceptLangStr = ctx.get('accept-language')
// https://www.npmjs.com/package/accept-language-parser
const parsed = acceptLanguageParser.parse(acceptLangStr)
const parsedISOSpec = parsed.find(p = p.code && p.region)

const lang = parsedISOSpec &&
  [parsedISOSpec.code, parsedISOSpec.region].join('-')
// 'en-US'

To anyone who may possibly use this package, feel free to open issues and support requests.