I never thought of Cowl as having a German accent or really any kind of accent. The only clue Cowl might be German is he knew the difference between Shubert's music Erlkönig and Goethe's poem, which provide the words used in the song. However, you don't have to be Austrian or German to know who those two people were. Harry has any affinity for classical music so he's heard of Franz Shubert and probably went to concerts where his music was played. Whether he's read much poetry or anything by Goethe is something we have no way of knowing, but I wouldn't be surprised if Harry is much better read than the average college graduate because he doesn't have a TV, radio or the internet.
I'm pretty sure Cowl's reference to Goethe was a quip jumping from Erlkonig to his even more famous work, the play Faust - as in, Cowl was alluding to having made a Faustian bargain himself, but it went over Harry's head at the time.
I doubt he has to be German to be familiar with those works though. Pretty much any wizard of European origin and sufficient age would presumably have been exposed to the range of classical music and literature. Most of the elderly, conventional Council types seem to make a point of flaunting that they're upper class - it probably would have been considered terribly vulgar not to educate their apprentices on culture as well as magic.
Cowl doesn't like "the bad man Kemmler".
That, again, falls under saying one thing in front of Kumori and another later when she's not listening. At Murphy's house, when she's inside searching for Bob and Cowl is talking to Harry outside, he makes it pretty clear that he has plenty of history with Kemmler and the other apprentices. He says knocking the power out while Grevane and Corpsetaker fought each other was par for the course - him doing the serious technical magic while they bickered - and that it was also normal that they'd present one face of smiling and getting along for the greater glory of Kemmler while planning to eliminate each other when the chance came.
He might not have been exactly an apprentice like the others. He might even have serious misgivings about Kemmler himself. But when he doesn't have to worry about shaking Kumori's delusion that he's in it for some greater good, he's quite free about admitting an association with Kemmler and company that goes beyond "nothing but disdain".
