much like everyone, robin struggled a lot in the aftermath of season 3 and one of the things that she did to help cope is fully throw herself into learning russian as a language. being able to translate a few sentences is what helped get her and the rest of the scoops troop into the russian base, but she didn’t know enough to really help the group after that ( would it have made a difference if she’d known more russian? she doesn’t know, but it easily could have done and that’s where the fixation comes from. she didn’t know enough to converse with the russian guard they encountered while sneaking around which is what prompted steve to throw himself at him, so that in itself is a good enough reason for her to learn without also considering the what ifs ). 

by the start of season 4, robin is not quite fluent yet, but definitely able to hold a conversation in the language ---- and naturally, the threat that comes after that from vecna isn’t russian in origin at all, but she now knows how to speak it, and to her that’s the important thing.