add signer user forms translations

app/controllers/application_controller.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class ApplicationController < ActionController::Base
+  BROWSER_LOCALE_REGEXP = /\A\w{2}(?:-\w{2})?/
+
   include ActiveStorage::SetCurrent
   include Pagy::Backend
 
@@ -32,6 +34,21 @@ class ApplicationController < ActionController::Base
 
   private
 
+  def with_browser_locale(&)
+    locale = request.env['HTTP_ACCEPT_LANGUAGE'].to_s[BROWSER_LOCALE_REGEXP].to_s
+
+    locale =
+      if locale.starts_with?('en-') && locale != 'en-US'
+        'en-GB'
+      else
+        locale.split('-').first.presence || 'en-GB'
+      end
+
+    locale = 'en-GB' unless I18n.locale_available?(locale)
+
+    I18n.with_locale(locale, &)
+  end
+
   def sign_in_for_demo
     sign_in(User.active.order('random()').take) unless signed_in?
   end

app/controllers/start_form_controller.rb

@@ -6,6 +6,7 @@ class StartFormController < ApplicationController
   skip_before_action :authenticate_user!
   skip_authorization_check
 
+  around_action :with_browser_locale, only: %i[show completed]
   before_action :load_template
 
   def show

app/controllers/submit_form_controller.rb

@@ -3,6 +3,7 @@
 class SubmitFormController < ApplicationController
   layout 'form'
 
+  around_action :with_browser_locale, only: %i[show completed]
   skip_before_action :authenticate_user!
   skip_authorization_check