@fluent/langneg 
@fluent/langneg provides an API for negotiating languages. It's part of
Project Fluent. Its main function is to provide functionality around the
Intl.Locale API with a focus on language negotiation, matching and
selection.
Installation
@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 https://projectfluent.org/fluent.js/langneg.
Strategies
The API supports three negotiation strategies:
filtering (default)
In this strategy the algorithm will try to match as many available locales as possible for each of the requested locale.
Example:
requested: ['de-DE', 'fr-FR'] available: ['it', 'de', 'en-US', 'fr-CA', 'de-DE', 'fr', 'de-AU']
supported: ['de-DE', 'de', 'fr', 'fr-CA']
matching
In this strategy the algorithm will look for the best matching available locale for each requested locale.
Example:
requested: ['de-DE', 'fr-FR'] available: ['it', 'de', 'en-US', 'fr-CA', 'de-DE', 'fr', 'de-AU']
supported: ['de-DE', 'fr']
lookup
In this strategy the algorithm will try to find the single best locale for the requested locale list among the available locales.
Example:
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 supports en-GB and en-US.