config.i18n.default_locale = :ja

Nameを入力してください
Emailを入力してください
Emailは不正な値です
Passwordを入力してください

config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.yml').to_s]

ja:
 activerecord:
   attributes:
       micropost:
         content: 投稿内容
         user:
         name: 名前
         email: メールアドレス
         password: パスワード
         password_confirmation: パスワード確認

require_relative 'boot'

require 'rails/all'

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module SampleApp
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 5.1
    config.time_zone = 'Tokyo'
    config.active_record.default_timezone = :local

    config.i18n.default_locale = :ja


    config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.yml').to_s]

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration should go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded.

  end
end

<gemfile>

source 'https://rubygems.org'


gem 'rails',        '5.1.6'
gem 'bcrypt',         '3.1.12'
gem 'faker',          '1.7.3'
gem 'will_paginate',           '3.1.6'
gem 'bootstrap-will_paginate', '1.0.0'
gem 'bootstrap-sass', '3.3.7'
gem 'puma',         '3.9.1'
gem 'sass-rails',   '5.0.6'
gem 'uglifier',     '3.2.0'
gem 'coffee-rails', '4.2.2'
gem 'jquery-rails', '4.3.1'
gem 'turbolinks',   '5.0.1'
gem 'jbuilder',     '2.7.0'
gem 'rails-i18n'

group :development, :test do
  gem 'sqlite3', '1.3.13'
  gem 'byebug',  '9.0.6', platform: :mri
end

group :development do
  gem 'web-console',           '3.5.1'
  gem 'listen',                '3.1.5'
  gem 'spring',                '2.0.2'
  gem 'spring-watcher-listen', '2.0.1'
end

group :test do
  gem 'rails-controller-testing', '1.0.2'
  gem 'minitest',                 '5.10.3'
  gem 'minitest-reporters',       '1.1.14'
  gem 'guard',                    '2.13.0'
  gem 'guard-minitest',           '2.4.4'
end

group :production do
  gem 'pg', '0.20.0'
end

# Windows環境ではtzinfo-dataというgemを含める必要があります
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]