lib/tasks/locale.rake

namespace :locale do
  desc "create locale file"
  task :create, [:model_name] => :environment do |task, args|
    model_name = args['model_name']
    const = model_name.camelize.constantize
    path = Pathname("config/locales/#{model_name.underscore}/ja.yml")
    FileUtils.mkdir_p(path.dirname)
    yml = ERB.new(<<-YML
ja:
  activerecord:
    models:
      <%= model_name.underscore %>: <%= model_name.camelize %>
    attributes:
      <%= model_name.underscore %>:
        <% const.column_names.each do |col| %>
        <%= col %>: <%= col %>
        <% end %>
YML
    ).result(binding)
    File.write(path, yml.gsub(/\n\s*\n/, "\n"))
    puts "created: #{path}"
  end

  task create_all: :environment do
    model_names = Dir['app/models/*.rb'].map{|path| path.match(/app\/models\/(.*)\.rb/); $1 }.map(&:camelize)

    model_names.each do |model_name|
      next if model_name == 'ApplicationRecord'
      Rake::Task["locale:create"].execute(Rake::TaskArguments.new([:model_name], [model_name]))
    end
  end
end

$ bin/rake locale:create[User]

で

config/locales/user/ja.yml

ja:
  activerecord:
    models:
      user: User
    attributes:
      user:
        id: id
        created_at: created_at
        updated_at: updated_at

のように作ってくれる