diff --git a/app/controllers/concerns/localized.rb b/app/controllers/concerns/localized.rb
index 5501b6793be..a9ea60f7dc6 100644
--- a/app/controllers/concerns/localized.rb
+++ b/app/controllers/concerns/localized.rb
@@ -17,12 +17,24 @@ module Localized
   end
 
   def default_locale
-    ENV.fetch('DEFAULT_LOCALE') do
-      user_supplied_locale || I18n.default_locale
-    end
+    request_locale || env_locale || I18n.default_locale
   end
 
-  def user_supplied_locale
-    http_accept_language.language_region_compatible_from(I18n.available_locales)
+  def env_locale
+    ENV['DEFAULT_LOCALE']
+  end
+
+  def request_locale
+    preferred_locale || compatible_locale
+  end
+
+  def preferred_locale
+    http_accept_language.preferred_language_from([env_locale]) ||
+      http_accept_language.preferred_language_from(I18n.available_locales)
+  end
+
+  def compatible_locale
+    http_accept_language.compatible_language_from([env_locale]) ||
+      http_accept_language.compatible_language_from(I18n.available_locales)
   end
 end
diff --git a/spec/controllers/concerns/localized_spec.rb b/spec/controllers/concerns/localized_spec.rb
new file mode 100644
index 00000000000..18fe508e44b
--- /dev/null
+++ b/spec/controllers/concerns/localized_spec.rb
@@ -0,0 +1,88 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe ApplicationController, type: :controller do
+  controller do
+    include Localized
+
+    def success
+      head 200
+    end
+  end
+
+  before do
+    routes.draw { get 'success' => 'anonymous#success' }
+  end
+
+  shared_examples 'default locale' do
+    context 'when DEFAULT_LOCALE environment variable is set' do
+      around do |example|
+        ClimateControl.modify 'DEFAULT_LOCALE' => 'ca', &example.method(:run)
+      end
+
+      it 'sets language specified by ENV if preferred' do
+        request.headers['Accept-Language'] = 'ca, fa'
+        get 'success'
+        expect(I18n.locale).to eq :ca
+      end
+
+      it 'sets available and preferred language if language specified by ENV is not preferred' do
+        request.headers['Accept-Language'] = 'ca-ES, fa'
+        get 'success'
+        expect(I18n.locale).to eq :fa
+      end
+
+      it 'sets language specified by ENV if it is compatible and none of available languages are preferred' do
+        request.headers['Accept-Language'] = 'ca-ES, fa-IR'
+        get 'success'
+        expect(I18n.locale).to eq :ca
+      end
+
+      it 'sets available and compatible langauge if language specified by ENV is not compatible none of available languages are preferred' do
+        request.headers['Accept-Language'] = 'fa-IR'
+        get 'success'
+        expect(I18n.locale).to eq :fa
+      end
+
+      it 'sets language specified by ENV if none of available languages are compatible' do
+        request.headers['Accept-Language'] = ''
+        get 'success'
+        expect(I18n.locale).to eq :ca
+      end
+    end
+
+    context 'when DEFAULT_LOCALE environment variable is not set' do
+      it 'sets default locale if none of available languages are compatible' do
+        request.headers['Accept-Language'] = ''
+        get 'success'
+        expect(I18n.locale).to eq :en
+      end
+    end
+  end
+
+  context 'user with valid locale has signed in' do
+    it "sets user's locale" do
+      user = Fabricate(:user, locale: :ca)
+
+      sign_in(user)
+      get 'success'
+
+      expect(I18n.locale).to eq :ca
+    end
+  end
+
+  context 'user with invalid locale has signed in' do
+    before do
+      user = Fabricate.build(:user, locale: :invalid)
+      user.save!(validate: false)
+      sign_in(user)
+    end
+
+    include_examples 'default locale'
+  end
+
+  context 'user has not signed in' do
+    include_examples 'default locale'
+  end
+end