diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 3e49e0ec..c7533db8 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -23,7 +23,7 @@ class ApplicationController < ActionController::Base
   private :render_not_authorized
 
   def set_locale
-    session[:locale] = params[:locale] if params[:locale]
+    session[:locale] = params[:custom_locale] || params[:locale] || session[:locale]
     I18n.locale = session[:locale] || I18n.default_locale
   end
   private :set_locale
diff --git a/spec/controllers/application_controller_spec.rb b/spec/controllers/application_controller_spec.rb
index 3d3a7ab9..0a3e3d83 100644
--- a/spec/controllers/application_controller_spec.rb
+++ b/spec/controllers/application_controller_spec.rb
@@ -40,9 +40,18 @@ describe ApplicationController do
     let(:locale) { :de }
 
     context 'when specifying a locale' do
-      it 'overwrites the session' do
-        expect(session).to receive(:[]=).with(:locale, locale.to_s)
-        get :welcome, locale: locale
+      context "using the 'custom_locale' parameter" do
+        it 'overwrites the session' do
+          expect(session).to receive(:[]=).with(:locale, locale.to_s)
+          get :welcome, custom_locale: locale
+        end
+      end
+
+      context "using the 'locale' parameter" do
+        it 'overwrites the session' do
+          expect(session).to receive(:[]=).with(:locale, locale.to_s)
+          get :welcome, locale: locale
+        end
       end
     end
 
diff --git a/spec/controllers/sessions_controller_spec.rb b/spec/controllers/sessions_controller_spec.rb
index d06ca7f8..7f85f03d 100644
--- a/spec/controllers/sessions_controller_spec.rb
+++ b/spec/controllers/sessions_controller_spec.rb
@@ -29,6 +29,7 @@ describe SessionsController do
   describe 'POST #create_through_lti' do
     let(:exercise) { FactoryGirl.create(:dummy) }
     let(:nonce) { SecureRandom.hex }
+    before(:each) { I18n.locale = I18n.default_locale }
 
     context 'without OAuth parameters' do
       it 'refuses the LTI launch' do
@@ -69,7 +70,8 @@ describe SessionsController do
     end
 
     context 'with valid launch parameters' do
-      let(:request) { post :create_through_lti, custom_token: exercise.token, oauth_consumer_key: consumer.oauth_key, oauth_nonce: nonce, oauth_signature: SecureRandom.hex, user_id: user.external_id }
+      let(:locale) { :de }
+      let(:request) { post :create_through_lti, custom_locale: locale, custom_token: exercise.token, oauth_consumer_key: consumer.oauth_key, oauth_nonce: nonce, oauth_signature: SecureRandom.hex, user_id: user.external_id }
       let(:user) { FactoryGirl.create(:external_user, consumer_id: consumer.id) }
       before(:each) { expect_any_instance_of(IMS::LTI::ToolProvider).to receive(:valid_request?).and_return(true) }
 
@@ -79,6 +81,12 @@ describe SessionsController do
         expect(session[:external_user_id]).to eq(user.id)
       end
 
+      it 'sets the specified locale' do
+        expect(controller).to receive(:set_locale).and_call_original
+        request
+        expect(I18n.locale).to eq(locale)
+      end
+
       it 'assigns the exercise' do
         request
         expect(assigns(:exercise)).to eq(exercise)