It could well suit his case to use a translator in order to come across as unsure of himself and not able to express himself properly in English, because he can then blame translation and misunderstandings for his future changes of stories, the slowing - down also gives him time to think and hesitate.
I don't see any report of his conversations with Libby about how to get to her home?
no stopping to check or turn around, no cruising the area to find her home, no wrong directions or wrong turns, did she direct him to the park?
And because the timing of his offence is so tight, I suspect even more strongly that he had prepared for this event - using 'running' as a reason for being out means he could have used it as a daytime check in the park, of how quickly he could cover the ground, then checking the area beforehand that same evening, and again after the event, because those before and after visits could have been to do something to decrease the time necessary for the actual offence.
He appears not to have even attempted to question Libby about where she lived, whether at the university, or in a flat, or a house, or a different town, with parents or boyfriend or husband or friends, or where she had been that evening (he could have taken her back there), so he wouldn't know how long she had been lost, or if she had already been reported missing and there were people already looking for her at the time he picked her up.
He needed to be very quick, as I recall his defence commented, and he knew this, and had planned to be very quick.