Using the link from Joe's answer and Flater's comments, I'll compile a single proper answer.
From the script (While correct German, the spelling is partly incorrect):
WONKA: Meine Herrschaften, schenken Sie mir ihre
aufmerksamkeit. [My friends (masters), please give me your
attention.] (Correct: Meine Herrschaften, schenken sie (plural!) mir ihre/Ihre Aufmerksamkeit)
WONKA: Sie kommen jetzt in den interessantesten und
gleichzeitig geheimsten raum meiner fabrik. [You have now
come to the most interesting and, at the same time, the most
secret room of my factory.] (Correct: Sie kommen jetzt in den interessantesten und gleichzeitig geheimsten Raum meiner Fabrik.)
WONKA: Meine Damen und Herren, der Inventing Room. [Ladies and
Gentlemen, The Inventing Room.]
Regarding why it is German, it's based in the Herr Doktor TV Trope.
In TV Land, a doctor or scientist is roughly 75% likely to be German or Austrian, complete with thick accent and often an entitled legacy. Increase to 98% for Mad Scientists. We can probably thank Sigmund Freud, Albert Einstein and Wernher von Braun.
Essentially, during and after WW2; many German scientists and scholars fled Germany (many of them to the US). They of course had a German accent when speaking. To the eye of an American citizen, all people with German accents (that they ever met) were almost always scientists. Over time, this starting being interpreted as "all Germans are scientists".
The movie tries to assert that Wonka is another crazy German scientist (and a successful one at that!)