"The man only appears at night."
Translation:Mężczyzna pojawia się tylko w nocy.
I guess, but it sounds kind of awkward.
If you were wondering why, it's probably because (technically) you can say that in your sentence "tylko" refers to the verb "pojawia się" so you imply something like: The man doesn't do almost anything except from appearing at night (only appears; does nothing else but appears). Doesn't make much sense. In the default translation, however, it refers to "noc" (only at night; never, except at night).
It's just part of the verb.
Some reflexive verbs are indeed reflexive in nature. They change the meaning in a predictable way: "myć się" (to wash oneself) vs just "myć" (to wash something/someone else). Some verbs have totally different, not even remotely connected meanings depending on whether they have "się" or not. "wydawać się" (to seem) vs "wydawać" (to spend money). And it's not that those with 'się' are in fact reflexive, they just have to use it.
And finally, some verbs just don't work without "się". "pojawiać" on its own has no meaning.
Because it's supposed to act on "at night". In English, I guess, both "only appears at night" and "appears only at night" can (although don't have to) mean the same. In Polish, placing "tylko" will definitely change the meaning - either "the only time in which he appears is night" or "the only thing he does is appearing at night".
I guess I'd better change the default to "appears only at night", so it won't be confusing anymore.