As ever in UI design, "it depends."
In this case the things you want to check include:
- What kind of users are you expecting? What technical level?
- What Web experience are they used to?
- What resources do you have for the localisation?
Technical users (e.g. developers) will most likely prefer the same layout on the site as other content.
For Arabic, while Web-savvy users are reasonably familar with left-to-right usage, it feels "wrong" and so your site will need to be quite compelling to make sure people stay. If you can change your layout then do.
For both Arabic and Chinese there are many customs and idioms that are quite different, especially when it comes to graphic design. Popular sites in particular look much "noisier" than is usual in the West: a site like stackoverflow.com would be very minimalist in these cultures.
Similarly be careful with things like lucky and unlucky numbers, colours with traditional associations, and suchlike. (This naturally goes everywhere: I could mention the comments we had from Northern Ireland about a very orange-branded theme that was once chosen for a site I worked on...)
What I'd recommend is that you allocate some of your translation budget to a local native-speaking UI consultant. They'd be able to give you proper advice on what bits of your site need to be localised as well as translated.