Translation:The fast snail is slower than a slow bird.
The first "langsamer" stands behind the noun it belongs to (die Schnecke), so it's not inflected langsam - langsamer - am langsamsten ("slow - slower - slowest").
The second "langsamer" stands before the noun it belongs to ((ein) Vogel), so it's an inflected form of "langsam" (slow). In this case, the masculine singular nominative strong ending (because of the indefinite article "ein"), so that is where the -er comes from.
If you answer only the word "fast", that's not a good translation of Die schnelle Schnecke ist langsamer als ein langsamer Vogel.
If you wrote more than one word, you'd have to post the entire sentence. You may be making some other mistake.
Even better would be a screenshot showing exactly what you typed and Duo's reaction (e.g. "right" or "wrong", and a suggested correction, if any).
Predicative adjectives do not take an ending in German for gender, number, or case.
Die Schnecke ist langsamer. "The snail is slower." thus only has the -er ending for comparison, but not also an -er ending for masculine nominative singular.
Just like der Mann ist groß, which is not der Mann ist *großer.
Because it's being compared with die schnelle Schnecke which is in the nominative case.
This lets you distinguish things such as "I love you more than him" which are ambiguous in English -- the "than him" would be als er or als ihn depending on whether you are comparing with the subject (I love you more than he loves you) or with the object (I love you more than I love him).
As an English teacher, I second what BenNew3 said.
Saying "It was he!" in response to "Who did this?" would sound really weird, yet it is the grammatically correct pronoun to use. So yes, it "was" originally as unambiguous as German. Having said that, I never think twice about saying "It was he who did that..." but I do avoid "It was he."
Cheers mizinamo for your answer regarding the German rule (and for all your answers everywhere.)
Nobody can tell you because you didn't tell us what you wrote -- nobody can see your translation.
Please write the entire sentence you used whenever you have a question. (And not just individual words; sometimes, the problem is with word order or with gender endings on some other word, rather than with the word you might think is the problem.)
I meant faster snail of course, didn't mean to trick you, but I get what you're saying, "schneller" would be wrong, it would have to be "schnellere". Thanks!
I have to admit that most of the time I guess at endings and have a bit of a feel for many of them, but keep forgetting which cases apply.