"I like your letter."
Translation:Aku suka suratmu.
Yes, it would be as Saya is the formal of Aku.
Whenever I've learned Indonesian in a formal setting, they would always teach us the formal sentence structures rather than informal because you would run the risk of insulting someone unintentionally if you spoke informally.
It's a little bit unfair, because in the first "question" they also used suratmu and not Anda. It's confusing now
yes sometimes the apps is lil bit unfair but, it's better if you know the two words.... try to put a warning