Inspect the relevant Cargo.toml, i18n.toml, library target, locale tree, and existing manager module. Preserve the repository's compatible release lines; manager versions can differ from framework versions.
Use the application's concrete runtime manager: embedded for general Rust, Dioxus for client/SSR, or Bevy for ECS and reactive UI.
Keep localizable types and define_i18n_module!() reachable from a library target. CLI derive discovery does not inspect binary-only types.
Give each package that owns messages its own configuration, fallback FTL, manager registration, and build-script asset tracking.
Use EsFluent for messages, EsFluentVariants for field or variant metadata, EsFluentLabel for type labels, and a unit-only EsFluent enum for selectors when it should also be a message.
For setup changes, run cargo es-fluent doctor. After changing message types, generate fallback FTL, translate it, and run the relevant CLI check.
Call i18n.localize_message(&message) through the explicit manager or framework context, and MyType::localize_label(&i18n) for type labels. Use fallible lookup only where the caller intentionally handles missing output.

Decision rules

Prefer the es-fluent facade and a concrete manager over direct dependencies on proc-macro or manager-core crates.
Use canonical BCP-47 locale directory names.
Treat the Cargo package name as the implicit package-local FTL domain. Additional domains do not reference another crate.
Use namespaces to split one domain into files, not to establish ownership.
Add es-fluent-build under [build-dependencies] for configured crates so locale changes rebuild and derives can compile-check the fallback catalog.
Keep the package-local missing_message_policy strict by default. Set it to fallback-str in i18n.toml only when normal runtime lookup should return snake_case source names after locale fallback fails; fallible lookup still returns None.
Preserve edited fallback translations in conservative generation mode. Preview cleanup and aggressive generation before allowing deletions.
In workspaces, validate from the root when the task spans multiple owner packages; use --package for intentionally narrow work.

References

Load only the reference needed for the request:

Public facades and setup: dependencies, configuration, release lines, and multi-crate ownership.
Runtime managers: embedded, Dioxus, and Bevy setup and context rules.
Derive and FTL patterns: attributes, generated IDs and arguments, choices, labels, variants, temporal values, namespaces, and domains.
CLI workflow: generation, checks, locale management, cleanup, structured output, and workspace selection.