"He seldom sleeps here."
Translation:Tady spí zřídka.
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.