I heard "slowly, please" and typed it. On reflection, I am sure the audio was "Langsam, bitte", no question. But from the sound, my brain heard "slowly please". On one level, that is quite encouraging w/re to my relearning German. Of course, it's not right per se. But when I started learning Spanish about 190 days ago, when I made a substitution like that, I received a friendly and encouraging pop-up: "Wrong language" or some such. I suggest you implement that in German as well.
I got it wrong because of the comma.
That would surprise me.
As you say, Duolingo ignores punctiation (with the exception of double quotes "..." and apostrophes, which are arguably more letter-like than punctuation-like).
Do you have a screenshot showing your answer being rejected purely for lack of a comma? If so, please upload it to a website somewhere (e.g. imgur) and tell us the URL. I'd be interested to see it.
"Bitte" means a lot of different things! You can say bitte for please, or for you're welcome. You'd say "Bitte" when handing something over, as in "Here you go". You can say "Bitte?" if you didn't understand something someone said. Or, if a waiter comes to your table, they might say "Bitte?" as in "May I help you?"
When you are a native speaker, or it is your second language you don't understand how your language is hard :D And then you try to learn some new languages and see that it is not easy at all xD I am glad that russian is my second language, I would suffer so much learning it, it seems hard =D