My EESU headcanons nobody asked for:
I read through the works of @dresden-syndrome and worked up some things about the lore, mainly concerning comrade Erhardt and Radím:
Radím has heavy Czech accent when speaking German. Despite his best efforts for proper pronunciation, the accent still peaks through whenever he speaks.
Günther finds it very cute, though he would never openly admit it. Atleast not to Radím. Instead he teases him about it. Especially his heavy R's.
Prior to his capture, Radím wanted to learn German as part of a plan to some day escape to Western Germany. The irony of his current situation makes him feel bitter. Now he’s learning German to communicate with his captor. To better serve the regime he worked so hard against. Knowing that he will never get more than a few feet away from Gunther, let alone getting away this god forsaken country.
Radím dreamed about learning English and maybe make his way to USA. However there wasn't much opportunity in Czechoslovakia to learn English. In this political minefield, attempting to learn it instantly got you put on a watchlist. As a counter-revolutionary he didn't want to draw more attention to him so he didn't pursue it.
The top choices for second language in Czechoslovakia were Russian or German. Radím learned a bit of both but focused more on German since he saw it as his potential ticket across the iron curtain.
German grammar has so many opportunities to make a mistake. Der/Die/Das, mein/meine. Not to mention the pronunciation and the unforgiving word order. This basically gives Günther a green card to punish his pet anytime he feels like it. This way its more painful for Radím because he knows this could have been avoided.
Russian is the main language used across the soviet union. So naturally, Günther as a government official must know it. He can both and read and understand Russian pretty well. He can speak Russian too but only does so when necessary, since its pretty tough for a German to speak Slavic language.
Because of that, he can understand quite a bit from what Radím is saying when he speaks Czech. Most of the time he pretends he pretends he doesn't. Its a wonderful opportunity to peek into the thoughts of his prisoner.
Günther probably also knows some Czech basics since he worked in Czechoslovakia. Its a neighbouring country and lot of Czechs and Slovaks work in Eastern Germany, since its seen as a more developed country.
Radím suspects that his owner knows a few words or phrases in Czech. He avoids swearing in Czech at all costs, since that's the first thing everyone learns. And a thing that you never forget. But it slips out so easy when you're under pressure.
Don't hesitate to correct me or share some of your thoughts about the lore! I love the story so much I can't stop thinking about it <3