From experience of teaching myself, you should at least know intermediate level Japanese or at least finish it as there are good amount of grammar not covered in an elementary course. But overall, while I haven't play much of Shiny Festa since it's not really that text heavy game and play mostly JRPGs, it's pretty easy. You might come across some kanji you might not know, so its best to familiarize yourself looking them up using radicals, which is parts that compose a whole character. There are usually some Kanji that are taught in Japanese schools, but not necessarily on the Kanji lists for the JLPT (Japanese Language Proficiency Test).
I haven't play Idolmaster recently since the auto text advance is not really that helpful. I probably will understand it a bit easier once I finish the intermediate textbook, but I still need a dictionary.