You're right insofar as no one would say "praeparieren fuer Krieg" in Germany. You're probably wrong that people in the US would understand the sentence in German either waythe level of language education here is APPALLING!!!

Blitzkriegs" notwithstanding, hardly anyone here even knows that means lightning WAR.
I hate to say it, but Christ they really must be clueless.
I got carded in a Win Dixie there in maybe 2011 I think it was..the girl meant well, although at the time I'm 50YO,(ok a little better kept than average) and she realized herself she should probably not have carded me, they have a rule that once ID is requested, they cannot have second thoughts and un-request it.
the assistant manager came..I said to him, half-jokingly "Im 50YO manJFK, I remember his assassination (I do, even though I was a toddler)..your President Nixon, Watergate, I went through all of that, I watched him resign live on TV , August 1974 , I was in junior High
..I watched all the Ali vs Joe Frazier vs George Foreman fights live on TV"
they had vaguely heard of JFK, but they had no idea who Nixon was or what Watergate wasthe boxers, I'm not sure if they knew about them.

99% of Americans, even educated ones, would not understand the words or phrase Wir prparieren fr Krieg. "Blitzkriegs" notwithstanding, hardly anyone here even knows that means lightning WAR.
More German language education: "Waffen" means weapons. So: Luftwaffe means "air-weapons". See? German is FUN.
And the Germans capitalize their Nouns.
HA

I was wondering about that, too.
By the way, I find it hard to believe that an intelligence service would send a message to a spy, even an encoded one, that contained vital information about the own side.

I think since it was a coded message they need to use less words as much as possible. No need to use full sentences when a few words can convey the same message. Full sentences will only take longer time to code/decode.

"Possible time frame or East vs West German turn of phrase?"
No.

Possible time frame or East vs West German turn of phrase?
Just saying, as it was a german production, presumably for germans (english speakers were already getting it subtitled) there would be no need to dumb it down for english speakers.