The code
Why the hell where the Russians transmitting it? Who was is for?
shareIt was intended to set up a rendezvous, communicate the place and time of a meeting to an agent out in the community. Better for your undercover operatives not to have any specialized gear in their possession. This way all they need is a shortwave radio. You could pick one up at Radio Shack and plenty of people had them. It's only because Robin happened to work in the mall, right over their installation where the meeting would be happening, that the code made sense to her.
With Hawkins Lab shut down no one would be trying to trace the transmission. Those few people who might hear it and recognize the language would most likely assume they were picking up a distant signal.
I wondered that too. It seems like that was not really a very secure way of doing business.
shareDepending on where your agents are embedded, it can be better and more secure than having them caught with a mil spec encrypted comm unit in their possession. It was probably a low power transmission limited to the town of Hawkins. What are the odds (now that the government presence is gone) of a random civilian a) Recognizing the language b) Realizing it's local c) Translating the message and d) Cracking the code ... in a little town like that?
shareBecause season 3's story sucks big time.
Are the Russians supposed to guess what the code means ?
Or did they tell them what it means ?
If they told them, what's the point of sending the coded message ?
You establish code language beforehand, that's how these things work. If for example a message starts with "The crow flies at dawn" one particular operative knows the following instructions are intended for them. The week is long might mean Tuesday for example, whereas the week is short may mean Monday or Wednesday. The day is long. The day is short. And so forth. Each represents a different day of the week.
Various code phrases involving the word 'meet' could indicate one of four or five spots designated for meetings. If that part of the code instead uses a phrase with the word 'leave' it might refer to one of their dead drop locations. Depending on what the undercover agent hears on their shortwave radio, they will take different actions, but no - they're not expected to guess what the code means. They're briefed on that in advance. It's your adversaries who are left guessing.