I know a bit of Japanese and understand most of the things on the P's and I can assure that you don't miss out on anything! Sure, being able to read and understand is nice, but it really is not necessary and all the conversations between them aren't important. You might as well just guess based on the context (for example, your tamagotchi was in school and writes a diary entry, you see her in class making a robot, you can pretty much guess that she is talking about how she made a robot) and you will be right. Or in the park; most of the tamas you meet there only give you tips (you have to go into the bath to stay clean) or when you meet the teachers they tell you to learn earnestly for tests. You can enjoy them without knowing any japanese.