To answer your first question, yes medium of instruction letter from your school or university can be used as an alternative to IELTS. However, keep in mind that it depends on the university. If you’re applying to a prestigious university, they might still ask you to take a language test such as the IELTS and TOEFL. About the Duolingo, again this depends on the university you’re applying to, but most universities have started accepting Duolingo since COVID so you can take that one. And as an additional tip, use Google to search for specific universities that do not require proficiency tests and/or accept Duolingo.