Add simple unit test for i18n system
user
```ruby
# frozen_string_literal: true

require 'test_helper'

class I18nTest < ActiveSupport::TestCase
  # Check that basic internationalization (i18n) functionality is working
  test 'i18n sanity check' do
    assert_equal 'Hello world', I18n.t(:hello, locale: :en)
    assert_equal 'Bonjour le monde', I18n.t(:hello, locale: :fr)
    assert_equal '你好，世界', I18n.t(:hello, locale: :'zh-CN')
  end
end
```
assistant