#localization #l10n #i18n #intl #internationalization


A localization system designed to unleash the entire expressive power of natural language translations

29 releases (16 breaking)

0.16.0 Jul 30, 2021
0.15.0 Feb 9, 2021
0.14.4 Jan 31, 2021
0.13.1 Sep 25, 2020
0.0.1 Jan 17, 2017

#123 in Internationalization (i18n)

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18,014 downloads per month
Used in 58 crates (22 directly)


1.5K SLoC


fluent-rs is a Rust implementation of Project Fluent, a localization framework designed to unleash the entire expressive power of natural language translations.

crates.io Build and test Coverage Status

Project Fluent keeps simple things simple and makes complex things possible. The syntax used for describing translations is easy to read and understand. At the same time it allows, when necessary, to represent complex concepts from natural languages like gender, plurals, conjugations, and others.



use fluent::{FluentBundle, FluentResource};
use unic_langid::langid;

fn main() {
    let ftl_string = "hello-world = Hello, world!".to_owned();
    let res = FluentResource::try_new(ftl_string)
        .expect("Failed to parse an FTL string.");

    let langid_en = langid!("en-US");
    let mut bundle = FluentBundle::new(vec![langid_en]);

        .expect("Failed to add FTL resources to the bundle.");

    let msg = bundle.get_message("hello-world")
        .expect("Message doesn't exist.");
    let mut errors = vec![];
    let pattern = msg.value
        .expect("Message has no value.");
    let value = bundle.format_pattern(&pattern, None, &mut errors);

    assert_eq!(&value, "Hello, world!");


The implementation is in pre-1.0 mode and supports Fluent Syntax 1.0, and Fluent API 0.14.. Consult the list of milestones for more information about release planning and scope.

FluentBundle, which is the main struct at the moment, is intended to remain a low level API. There is a number of higher level APIs like fluent-resmgr and fluent-fallback intended to wrap around it and provide better ergonomics for bindings and direct usage. Those higher level APIs are not mature yet enough to be included in this package, but will get added once they are.

Local Development

cargo build
cargo test
cargo bench
cargo run --example simple-app

When submitting a PR please use cargo fmt (nightly).

Learn the FTL syntax

FTL is a localization file format used for describing translation resources. FTL stands for Fluent Translation List.

FTL is designed to be simple to read, but at the same time allows to represent complex concepts from natural languages like gender, plurals, conjugations, and others.

hello-user = Hello, { $username }!

Read the Fluent Syntax Guide in order to learn more about the syntax. If you're a tool author you may be interested in the formal EBNF grammar.

Get Involved

fluent-rs is open-source, licensed under the Apache License, Version 2.0. We encourage everyone to take a look at our code and we'll listen to your feedback.


We'd love to hear your thoughts on Project Fluent! Whether you're a localizer looking for a better way to express yourself in your language, or a developer trying to make your app localizable and multilingual, or a hacker looking for a project to contribute to, please do get in touch on the mailing list and the IRC channel.


~33K SLoC