Localizing ActionMailer::Preview

config-application.rb

class Application < Rails::Application
  ...
  
  # The easiest way : update the default locale and restart
  config.i18n.default_locale    = :fr
  
  ...
end

multiple-previews.rb

class MyMailerPreview < MailerPreview
  def notify_error
    MyMailer.notify_error(MyModel.last)
  end

  def notify_error_fr
    I18n.with_locale(:fr) do
      notify_error
    end
  end
end

my-solution.rb

class Rails::MailersController < Rails::ApplicationController
  prepend_view_path File.expand_path('../../../../lib/templates', __FILE__)
  
  before_action :set_locale
  
  def set_locale
    I18n.locale = params['locale'] || I18n.default_locale
  end
end

template.erb

    <% unless @email.attachments.nil? || @email.attachments.empty? %>
      <dt>Attachments:</dt>
      <dd>
        <%= @email.attachments.map { |a| a.respond_to?(:original_filename) ? a.original_filename : a.filename }.inspect %>
      </dd>
    <% end %>

    <% if @email.multipart? %>
      <dd>
        <select onchange="document.getElementsByName('messageBody')[0].src=this.options[this.selectedIndex].value;">
          <option <%= request.format == Mime::HTML ? 'selected' : '' %> value="?part=text%2Fhtml&amp;locale=<%= I18n.locale %>">View as HTML email</option>
          <option <%= request.format == Mime::TEXT ? 'selected' : '' %> value="?part=text%2Fplain&amp;locale=<%= I18n.locale %>">View as plain-text email</option>
        </select>
      </dd>
    <% end %>
  </dl>
</header>

<iframe seamless name="messageBody" src="?part=<%= Rack::Utils.escape(@part.mime_type) %>&amp;locale=<%= I18n.locale %>"></iframe>

the-naive-approach.rb

class MailerPreview < ActionMailer::Preview
  before_action :set_locale
  
  def set_locale
    I18n.locale = params['locale'] || I18n.default_locale
  end
end