Add i18n support

i18n.toml

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+fallback_language = "en"
+
+[fluent]
+assets_dir = "i18n"

i18n/en/cosmic_workspaces.ftl

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+new-workspace = New Workspace

src/localize.rs

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+// Copyright 2023 System76 <[email protected]>
+// SPDX-License-Identifier: GPL-3.0-only
+
+use i18n_embed::{
+    fluent::{fluent_language_loader, FluentLanguageLoader},
+    DefaultLocalizer, LanguageLoader, Localizer,
+};
+use once_cell::sync::Lazy;
+use rust_embed::RustEmbed;
+
+#[derive(RustEmbed)]
+#[folder = "i18n/"]
+struct Localizations;
+
+pub static LANGUAGE_LOADER: Lazy<FluentLanguageLoader> = Lazy::new(|| {
+    let loader: FluentLanguageLoader = fluent_language_loader!();
+
+    loader
+        .load_fallback_language(&Localizations)
+        .expect("Error while loading fallback language");
+
+    loader
+});
+
+#[macro_export]
+macro_rules! fl {
+    ($message_id:literal) => {{
+        i18n_embed_fl::fl!($crate::localize::LANGUAGE_LOADER, $message_id)
+    }};
+
+    ($message_id:literal, $($args:expr),*) => {{
+        i18n_embed_fl::fl!($crate::localize::LANGUAGE_LOADER, $message_id, $($args), *)
+    }};
+}
+
+// Get the `Localizer` to be used for localizing this library.
+#[must_use]
+pub fn localizer() -> Box<dyn Localizer> {
+    Box::from(DefaultLocalizer::new(&*LANGUAGE_LOADER, &Localizations))
+}