"양이 건너편에서 자요."
Translation:The sheep sleeps on the opposite side.
Because that's not how you say that sentence in English. The only way your sentence sounds correct is if you're emphasising the word opposite, as if there was a sheep on this side that does not sleep. The korean sentence doesn't have such an emphasis so yours is a bad translation.
Actually, Conniestar's sentence is just fine in English, although it is a matter of the perspective of the speaker and what they are trying to emphasize, as you noted. "The sheep on the opposite side sleeps" could emphasize a contrast to another sheep nearby as you proposed, but it also could also emphasize the speaker's view of the sheep as distant from themselves, such as being separated by a major land feature like a river or mountain range, possibly not even within view. However, in English "The sheep sleeps on the opposite side" is more likely to be used when the speaker and the sheep are nearer and within view of each other, just on opposite sides of a shared object or locale, such as a bed or a street. Obviously, this is a pretty subtle nuance in English and since I'm really new to Korean I have no idea what the original Korean sentence devoid of any context was emphasizing. Nevertheless, to belittle conniestar's sentence as a bad translation seems a bit severe, no?