RoR 4 How to save extra options with has_many and form.select?

Question

Ruby on Rails

There are models:

# Посты
class Post < ActiveRecord::Base
  has_many :post_categories
  has_many :categories, :through => :post_categories
end

# Категории
class Category < ActiveRecord::Base
  has_many :post_categories
  has_many :posts, :through => :post_categories
end

# Поля таблицы
# post_id,
# category_id, 
# lang  - string(2) например "en" или "ru"
class PostCategory < ActiveRecord::Base
  has_one    :post
  has_one    :category
end

There is a form for saving/adding a new post

<div class="col-sm-5">
        <%= f.select :category_ids, options_for_select(Category.all.collect { |c| [c.title, c.id] },@post.categories.collect { |c| c.id }), {:include_hidden => false}, {:multiple => true, :class => 'form-select form-control-static', :style => 'width: 100%'} %>
</div>

In which I specify :category_ids as a variable through which the categories to which the post may belong are saved.
There is a controller

# Сохранение нового поста
def create
   @post = Post.new(post_params)
   #                                 ^^^
   # При создании поста я знаю, как передать 3 параметр в таблицу связей, потому что 
   # уже имею все данные в связях
   @post.post_categories.each do |с|
      с.lang = 'ru'
    end

   if @post.save
      redirect_to post_path(@post)
    else
      render 'new'
    end
end
def update
    @post = Post.find(params[:id])
    # Как передавать параметры lang сюда, при обновлении категорий у поста?
    if @post.update(post_params)
      redirect_to post_path(@post)
    else
      render 'edit'
    end
end

private
    def post_params
      params.require(:post).permit(:title, {:category_ids => []})
    end