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fluent-langneg is an API for negotiating languages. It's part of Project Fluent, a localization framework designed to unleash the expressive power of the natural language.


fluent-langneg can be used both on the client-side and the server-side. You can install it from the npm registry or use it as a standalone script.

npm install fluent-langneg

How to use

import { negotiateLanguages } from 'fluent-langneg';
const supportedLocales = negotiateLanguages(
  navigator.languages,       // requested locales 
  ['de', 'en-US', 'pl'],     // available locales 
  { defaultLocale: 'en-US' }

The API reference is available at


The API supports three negotiation strategies:

filtering (default)

In this strategy the algorithm will look for the best matching available locale for each requested locale.


requested: ['de-DE', 'fr-FR'] available: ['it', 'de', 'en-US', 'fr-CA', 'de-DE', 'fr', 'de-AU']

supported: ['de-DE', 'fr']


In this strategy the algorithm will try to match as many available locales as possible for each of the requested locale.


requested: ['de-DE', 'fr-FR'] available: ['it', 'de', 'en-US', 'fr-CA', 'de-DE', 'fr', 'de-AU']

supported: ['de-DE', 'de', 'fr', 'fr-CA']


In this strategy the algorithm will try to find the single best locale for the requested locale list among the available locales.


requested: ['de-DE', 'fr-FR'] available: ['it', 'de', 'en-US', 'fr-CA', 'de-DE', 'fr', 'de-AU']

supported: ['de-DE']

API use:

let supported = negotiateLanguages(requested, available, {
  strategy: 'matching',

Likely subtags

Fluent Language Negotiation module carries its own minimal list of likely subtags data, which is useful in finding most likely available locales in case the requested locale is too generic.

An example of that scenario is when the user requests en locale, and the application supportes en-GB and en-US.

Unicode CLDR maintains a complete list of likely subtags that the user can load into fluent-langneg to replace the minimal version.

let data = require('cldr-core/supplemental/likelySubtags.json');
let supported = negotiateLanguages(requested, available, {
  likelySubtags: data.supplemental.likelySubtags

Learn more

Find out more about Project Fluent at, including documentation of the Fluent file format (FTL), links to other packages and implementations, and information about how to get involved.