2) You need a "B2-certificate" as a proof that you can speak/read/write German. You can follow lessons for instance at the "Goethe-institut" with exams.
Deutsch lernen, Kultur erleben - Goethe-Institut
But simply write an email to Lufthansa. They are willing to assist you in answering your questions. And yes, just write the email in English and explain that you are improving your German...
Good luck! Lufthansa-scheme is probably the best training with lowest financial risks in the world and one of the best job opportunities...! It's worth all the effort.