I18n Patterns for Rails 8

Overview

Rails I18n provides internationalization support:

• Translation lookups

• Locale management

• Date/time/currency formatting

• Pluralization rules

• Lazy lookups in views

Quick Start

config/application.rb

config.i18n.default_locale = :en config.i18n.available_locales = [:en, :fr, :de] config.i18n.fallbacks = true

Project Structure

config/locales/ ├── en.yml # English defaults ├── fr.yml # French defaults ├── models/ │ ├── en.yml # Model translations (EN) │ └── fr.yml # Model translations (FR) ├── views/ │ ├── en.yml # View translations (EN) │ └── fr.yml # View translations (FR) ├── mailers/ │ ├── en.yml # Mailer translations (EN) │ └── fr.yml # Mailer translations (FR) └── components/ ├── en.yml # Component translations (EN) └── fr.yml # Component translations (FR)

Locale File Organization

Organize locale files by domain: models/ , views/ , mailers/ , components/ .

• Models: activerecord.models , activerecord.attributes , activerecord.errors

• Views: nested under controller name and action (e.g. events.index.title )

• Shared: common.actions , common.messages , common.date.formats

• Components: components.<component_name>.<key>

See locale-files.md for complete YAML examples for models, views, shared keys, and components.

Usage Patterns

Key Principles

• Use lazy lookup in views: t(".title") resolves to "events.index.title"

• Use _html suffix for strings containing HTML markup

• Use I18n.l (localize) for dates, times, and numbers — not I18n.t

• Use I18n.t with full key path in models, services, and presenters

• Pass dynamic values via interpolation: t(".greeting", name: user.name)

In Views

<h1><%= t(".title") %></h1> <%= link_to t(".new_event"), new_event_path %> <p><%= t(".welcome", name: current_user.name) %></p> <p><%= t(".intro_html", link: link_to("here", help_path)) %></p>

In Controllers

redirect_to @event, notice: t(".success")

In Models/Presenters

I18n.t("activerecord.attributes.event/statuses.#{status}") I18n.l(event_date, format: :long)

See usage-patterns.md for full examples including presenters, components, date/currency formatting, and pluralization.

Locale Switching

URL-Based Locale

config/routes.rb

Rails.application.routes.draw do scope "(:locale)", locale: /en|fr|de/ do resources :events end end

app/controllers/application_controller.rb

class ApplicationController < ActionController::Base around_action :switch_locale

private

def switch_locale(&action) locale = params[:locale] || I18n.default_locale I18n.with_locale(locale, &action) end

def default_url_options { locale: I18n.locale } end end

User Preference Locale

class ApplicationController < ActionController::Base around_action :switch_locale

private

def switch_locale(&action) locale = current_user&.locale || extract_locale_from_header || I18n.default_locale I18n.with_locale(locale, &action) end

def extract_locale_from_header request.env['HTTP_ACCEPT_LANGUAGE']&.scan(/^[a-z]{2}/)&.first end end

Testing I18n

• Raise on missing translations in spec/rails_helper.rb

• Use i18n-tasks gem to detect missing and unused keys

• Write view translation specs to assert rendered content

See testing.md for complete spec examples and i18n-tasks configuration.

Best Practices

DO

Use nested structure matching view paths

en: events: index: title: Events show: title: Event Details

Use interpolation for dynamic content

en: greeting: "Hello, %{name}!"

Use _html suffix for HTML content

en: intro_html: "Welcome to <strong>our app</strong>"

DON'T

Don't use flat keys

en: events_index_title: Events # BAD

Don't hardcode in views

<h1>Events</h1> # BAD - use t(".title")

Don't concatenate translations

t("hello") + " " + t("world") # BAD

Checklist

• Locale files organized by domain (models, views, etc.)

• All user-facing text uses I18n

• Lazy lookups in views (t(".key"))

• Pluralization for countable items

• Date/currency formatting localized

• Locale switching implemented

• i18n-tasks configured

• Missing translation detection in tests

• Fallbacks configured

References

• locale-files.md — YAML locale file examples for models, views, shared keys, and components

• usage-patterns.md — Usage examples in views, controllers, models, presenters, components, and formatting

• testing.md — RSpec specs and i18n-tasks for translation coverage