I believe you are indenting those as they should be indented? Also, it should be one:
and other:
, not many:
x_days:
one: "1 dias"
other: "%{count} dias"
UPDATE:
The following works for me:
en:
hello: "Hello world" # irrelevant
datetime:
distance_in_words:
x_days:
one: "1 dias"
other: "%{count} dias"
Note that indentation is a key here.