Deny /implement without execution environment

Also, fix a smaller typo in the locales

app/controllers/exercises_controller.rb

@@ -443,6 +443,7 @@ class ExercisesController < ApplicationController
     if current_user.admin? || current_user.teacher?
       redirect_to(@exercise, alert: t('exercises.implement.unpublished')) if @exercise.unpublished?
       redirect_to(@exercise, alert: t('exercises.implement.no_files')) unless @exercise.files.visible.exists?
+      redirect_to(@exercise, alert: t('exercises.implement.no_execution_environment')) if @exercise.execution_environment.blank?
     else
       render_not_authorized
     end

app/policies/exercise_policy.rb

@@ -31,7 +31,7 @@ class ExercisePolicy < AdminOrAuthorPolicy
 
   %i[implement? working_times? intervention? search? reload?].each do |action|
     define_method(action) do
-      return no_one unless @record.files.visible.exists?
+      return no_one unless @record.files.visible.exists? && @record.execution_environment.present?
 
       admin? || teacher_in_study_group? || author? || (everyone && !@record.unpublished?)
     end