Like the British airmen in ’Allo, 'Allo. Jolly good! 
Of course you can, as long as it is clear that you are taking the piss at the English! Or, even better, you are using the French to take the piss at the English. Which makes it hilarious indeed.
As @Pardel-Lux said, “Alter Knabe” is a perfectly cromulent rendering of “Old boy”. Maybe the Swedish girl was thinking of “Alter Schwede” (Old Swede), which has a different connotation and may be a bit off putting for a Swedish person.
I don’t think my Swedish friend (who was multilingual) meant it wasn’t grammatically possible, just that it’s not a common saying in German.
Unless, of course, you are “taking the piss” (a great expression!) out of the British.
There were other phrases translated literally that I thought sounded funny, even though I’m not a native German speaker.
One joke that went completely over my head at first was in Asterix at the Olympic Games:
Obelix is afraid someone is going to steal their magic potion. He’s bewildered when Asterix tells him not to worry.
ASTERIX: Theft of cauldrons is not a crime among the Hellenes.
OBELIX: (Talking to his dog) Do you understand the cauldron laws in these parts, Dogmatix?