Tonight I read Japanese to the girls for the first time. It was Momo Tarou, a children's classic adventure fairy tale. They responded about the same as they do in English.
Its been a few years since I've even bothered to try to read Japanese, so it was rough going even though it was a children's book. Every word came out as a separate entity rather than smooth, natural speech. Ah well. At least we could get through it all in one sitting.
Overall I prefer to read to the girls in English. At this point in their unborn lives, it doesn't really matter anyway since they can't understand a word I say in any language.