I am writing a World War II novel from the POV of a German colonel (among others). In the case of the colonel, is it appropriate to use the English idiom "top brass" in his internal dialogue? To me, it seems to be the fastest and most descriptive manner of conveying the intended meaning to my readers.
This example would seem inappropriate to someone familiar with Nazi Germany.
"The brass" is used in the US-American sphere to refer to the highes ranking military officers because they are wearing rank insignia made of brass. This was not the case in the Wehrmacht. General-level officers had silver insignias on a red/yellow/white background. But there were some lower officer ranks which had brass insignia. The Wehrmacht equivalent of a colonel ("Oberst") was incidentally the highest rank with brass insignia. So a Nazi colonel thinking about "the brass" would be thinking about soldiers of the same rank as himself or lower.
Also, Nazi officers were highly indoctrinated and obsessed with discipline and formality. Such an informal idiom would seem out of place in internal monologue of one (unless he is demoralized and broken by the horrors of war and turned cynical). You might want to look up which command instance he would think of specifically and use it. The result could be a phrase like:
The Armeeoberkommando still hadn't approved my request for more soldiers.