I think it could suit his purpose to come across in translation as hesitant and not able to express himself properly in English
I think it would suit him to have a translator that is not too accurate because that could allow him to claim misunderstandings, and to change his stories in the future, it makes him appear not so confident and sure of himself, which would be in his favour.
He doesn't seem to have reported any conversations with Libby about where she lived and how to get there, nor asking her name or where she had been that evening (he could have taken her back there) He didn't ask if she lived with her parents, husband, boyfriend, friend, nor whether she lived in Hull, or at the university or in a house or a flat or shared building etc.
At the time he picked her up he did not know how long she had been missing, lost, wandering, so he didn't know if she had been reported as missing nor if people or police were already looking for her. He did know that he needed to be quick, and I think his earlier and then later visit to the park were in some way to facilitate being as quick as possible once he had got hold of someone that night. His explanation of going running would have facilitated daylight visits to the park to see how quickly that ground could be covered, his own defence said someone would need to be very quick? to fit the timescale, but not impossible.
He knew that he needed to be very quick once he had someone in his car and he had planned for that.
(I hope I post this in the right place this time)!