"He seldom sleeps here."
Translation:Tady spí zřídka.
"On zřídka tady spí" couldn't be valid? Word order in Czech is more flexible than in English, for instance.
It doesn't sound very natural, to be honest. You don't use personal pronouns in Czech that much, so I would drop the on Also, keep in mind that you usually place the information you want to stress at the end of the sentence.
Zřídka tady spí. = 'He rarely SLEEPS here.'
Tady spí zřídka. = 'He RARELY sleeps here.