I don't know why, but after adding a translation, many steps appear red in the top bar.

where is the solution

The solution to every task is not given as a whole block that when copy-pasted will just work, rather as an example block of code under the named as Hint dropdown subsection on the left side just under the Task section. This example may use different variable names from those in the task's source but explicitly says what to use and is trivial to transfer to the implementation of the task.
Also, there are tasks that continue over several parts such as the reduction program implementation where you will see the final answer/result at the final part.

Not sure but I think this is using CodePlay's implementation. I got here from a link on their site. https://www.codeplay.com/

Just exploring.