gistfile1.txt

TL:DR - after changing the language, if user submits invalid values for Post model, the validation error is still thrown in the previous locale. 

Extended version with code:

I use a button to change the I18n.locale, which also sets the locale in session[:locale]. Everything works fine, every view is translated, flash messages as well, devise views, etc.

The problem I stepped upon, is that the Post model validation errors, after changing the locale with the button, are still sending messages translated to the old language (everything else, flash messages, devise views, and other views are properly translated). Refreshing the site doesn't fix it. 

E.g.
Model validations:

validates_length_of :description, :minimum => 5, :maximum => 500, :too_long => I18n.t('posts.too_long'), :too_short => I18n.t('posts.too_short')

Reproducing the error:

post = Post.new(post_params)
post.user_id = current_user.id
if post.save
  render :json => { :status => 0, :messages => ["OK"] }
else
  render :json => { :status => 1, :messages => post.errors.messages[:description] }
end

If the post doesn't save, the error is rendered in the old language. 

Locale fragments:
en.yml
  posts:
    too_short: "The message you're trying to send is too short."
    too_long: "The message you're trying to send is too long."
pl.yml
  posts:
    too_short: "Wiadomość którą próbujesz wysłać jest za krótka."
    too_long: "Wiadomość którą próbujesz wysłać jest za długa."

POST action which is being pointed by a button, to change the locale:
  def change_locale
    if session[:locale] == 'pl'
      I18n.locale = :en
      session[:locale] = 'en'
    else
      I18n.locale = :pl
      session[:locale] = 'pl'
    end
  end

And the application_controller action that keeps track of the locale: (used with a before_action filter)

  def set_locale
    if session[:locale] == 'pl'
      I18n.locale = :pl
    elsif session[:locale] == 'en'
      I18n.locale = :en
    end
  end