Yes, you are right. But a native English speaker would probably use "slower" since "more slowly" COULD, but not necessarily, be taken as an implied criticism. It's really six of one, half dozen of the other. Sometimes when I'm watching TV, a person will speak so fast that I'm tempted to say "speak more slowly; I can't understand what you're saying!"
Literally, it is langsam, langsamer = slow, slower. Making a comparative as "more/less + adjective/adverb" is possible in English, but not in German - it's gramatically incorrect. For that reason is always recommended to avoid translating German comparatives to English that way, because it may distort the meaning of whole sentence. That is exactly the problem in this case:
1) "Can you please speak slowly?" - OK in meaning, but missing a German comparative.
2) "Can you please speak MORE slowly?" - translates a comparative, but confuses the meaning: it's like the person already speaks slowly and is asked to speak at even slower pace.
3) "Can you please speak slower?" - is middle of the road thing: translates a comparative and does not distort the original meaning, although comes as somehow shady (but not incorrect?) with English grammar as slower is rather seen as an adjective than an adverb.
It doesn't sound great to me... "slowly" is an adverb, but "slower" is usually used as an adjective. To describe an action (e.g. how someone speaks), you need an adverb. However, Merriam Webster dictionary mentions that there's a contentious history of "slow/slower" being used as an adverb - so perhaps it should be accepted.
That is incorrect. English adjectives and adverbs tend to have the same comparative degrees:
He is good, but I am better. Sleep well and feel better.
I walk slowly but he walks even slower.
https://www.oxfordlearnersdictionaries.com/definition/english/slow_2 - this entry is specifically for "slow" as an adverb.
While "more slowly" is indeed acceptable in this context, it should not be the only version acceptable here (it is now).
"Can you please speak slower" is the common way of saying that in the midwest US. I don't care that much about what Merriam Webster says if my translation is the exact phrase that would be used by 99% of the people in the US.
It seems like there are multiple people at Duo deciding what is an allowable correct answer for both English to German and German to English, but they don't apply the same criteria. Sometimes a colloquial phrase is allowed but other times it's a strict grammatically correct translation.
Check the comparative forms for yourself.
more slowly is not a good English translation. The only people who speak English that would say More Slowly would be a child or someone who does not speak English very well. The correct English translation would be: Can you please speak slower? I see this has been stated below and again DUO refuses to listen to native English speakers.