How can I confirm the language is English before I rent?
The languages section on the movie Info page is indeed the indication to look for. When it states “Primary: English (Dolby 5.1), Additional: […] Japanese (Dolby 5.1) […]”, then that is what is should have available for your country or region.
The languages of the trailer is not an indication of options.
If that information would be incorrect in some unfortunate instance, then you can ask for a refund on those grounds. But I haven’t seen, or heard of, errors in the Languages information yet.