Why GO is not accepted?
I've added that. :)
should be a short "a" in "långsammare": http://sv.forvo.com/search-sv/l%C3%A5ngsammare/
Walk slower isn't accepted?
Slower is an adjective, right? And I guess you need an adverb here.
It depends which version of English one uses, to quote:
at a slow pace; slowly: the train went slower and slower | [in combination] : a slow-moving river.