Localization Rails

1)

# Gemfile
gem 'rails-i18n'

bundle

2)
bin/rails g migration add_language_to_users language

class AddLanguageToUsers < ActiveRecord::Migration[6.1]
  def change
    add_column :users, :language, :string
  end
end

bin/rails db:migrate

3)
config/application.rb

 config.time_zone = "Kyiv" # 'Paris', 'Moscow' 
 config.i18n.available_locales = %i(en ru)
 config.i18n.default_locale = :en
 config.i18n.fallbacks = true
 
 4)
app/controllers/application_controller.rb
before_action :set_locale

  def set_locale
    if user_signed_in? && !current_user.language.blank?
      I18n.locale = current_user.language
    else
      I18n.locale = params[:lang] || locate_from_header || I18n.default_locale
    end
  end
### Alternative 'set_locale' method for non-login user #####
    def set_locale
    if user_signed_in? && !current_user.language.blank?
      I18n.locale = current_user.language
    else
      cookies[:my_locale].clear unless !cookies[:my_locale].blank? && I18n.available_locales.include?(cookies[:my_locale].to_sym)

      I18n.locale =  params[:lang] || cookies[:my_locale] || locate_from_header || I18n.default_locale
      cookies.permanent[:my_locale] = I18n.locale
    end
  end
### Alternative for non-login user #####

  
  def default_url_options
    return {} if I18n.locale == I18n.default_locale

    { lang: I18n.locale } # { lang: ((I18n.locale == I18n.default_locale) ? nil : I18n.locale) }
  end
  
  private
  
  def locate_from_header
    request.env.fetch('HTTP_ACCEPT_LANGUAGE', '').scan(/[a-z]{2}/).first
  end

5) 
Navigator menu
app/views/shared/_navigation.html.erb
<ul>
  <% unless user_signed_in? %>
    <% if I18n.locale == :ru %>
      <li class="nav-item"><%= link_to '?lang=en'.prepend(request.path), class: 'nav-link' do %>
          <i class="fas fa-toggle-on"><span class='fw-normal'> English</span></i>
        <% end %>
      </li> 
    <% else %>
      <li class="nav-item"><%= link_to '?lang=ru'.prepend(request.path), class: 'nav-link' do %>
          <i class="fas fa-toggle-on"><span class='fw-normal'> Русский</span></i>
        <% end %>
      </li>
    <% end %>
 </ul>
 <!-- request.path + "?lang=ru" ? current_uri = request.env['PATH_INFO'] or request.path_info or request.url or request.path 
or request.env['ORIGINAL_FULLPATH'] or request.env['REQUEST_URI']-->
  <% end %>

6)
Add to:
config/locales

en.yml
ru.yml
..

PS
Pluralization Rules:
for En (simple) -
en:
  global:
    forms:
      submit: Submit
      messages:
        errors:
          one: "One error prohibited this feedback from being saved"
          other: "%{count} errors prohibited this feedback from being saved"

for Russian -
ru:
  global:
    forms:
      submit: Отправить
      messages:
        errors:
          one: "Не удалось сохранить отзыв! Найдена одна ошибка:"
          few: "Не удалось сохранить отзыв! Найдены %{count} ошибки:"
          many: "Не удалось сохранить отзыв! Найдено %{count} ошибок:"
          other: "Не удалось сохранить отзыв! Найдена %{count} ошибка:"
  
  Sample:
  <h2><%= t 'global.forms.messages.errors', count: feedback.errors.count %></h2>
 
 Date
 <!-- views/feedbacks/_feedback.html.erb -->
<article>
  <em>
    <%= tag.time l(feedback.created_at, format: :long), datetime: feedback.created_at %><br>
    <%= t 'global.feedback.posted_by', author: feedback.author %>
  </em>
  <!--... -->
</article>


More 
def extract_locale
  parsed_locale = params[:locale]
  I18n.available_locales.map(&:to_s).include?(parsed_locale) ? parsed_locale : nil
end

def change_locale
  l = params[:locale].to_s.strip.to_sym
  l = I18n.default_locale unless I18n.available_locales.include?(l)
  cookies.permanent[:my_locale] = l
  redirect_to request.referer || root_url