# config/routes.rb
Rails.application.routes.draw do
  root to: "home#welcome"
end

# app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  before_action :apply_locale
 
  def apply_locale
    I18n.locale = params[:locale] || I18n.default_locale
  end
end

# app/controllers/welcome_controller.rb
class WelcomeController < ApplicationController
  def welcome
    flash[:notice] = t(:flash_msg)
  end
end

# app/views/welcome/welcome.html.erb
<h1>This is the world of rails</h1> # = > This is the world of rails
<p><%= flash[:notice] %></p> # = > This is a Flash Message

To apply translation, there is a 't' helper available in rails for translation. For applying this we need to pass translation keys in place of actual string and need to map them. In your file you can define the keys as:

# config/locales/en.yml
en:
  rails_world: This is the world of rails
  flash_msg: This is a Flash Message
 
# config/locales/pirate.yml
pirate:
  rails_world: This be a world o' rails
  flash_msg: This be a Flash Message

Then if you hit http://www.localhost:3000, you will get this page:

and if you hit http://www.localhost:3000?locale=pirate, you will get this:

You can also pass variables inside translation messages. You can do it like:

#inside en.yml
en:
  flash_msg: This is a Flash Message
  rails_world: This is a world of rails
  first_message: "Hi %{username}, %{message}"

#inside view welcome.html.erb
<h1><%=t 'first_message', username: "Reader", message: "Did this example worked ?" %></h1>
<h2><%=t :rails_world %></h2>
<p><%= flash[:notice] %></p>

A open source git project of this example can be obtained here:
Rails Translation Example

Other articles related to internationalization are available here:

• Configuring i18N Api in rails for internationalization
• Setting rails locale from domain name
• Setting the Locale from the URL Params
• Setting the Locale from Client Requests