My advice is to go with a turboduo. The PC-FX will have nothing to do with learning japanese. I know. Some years ago I was learning japanese and bought a PC-FX. I didn't have much fun and wondered why the hell it seemed everyone liked the PC-FX. I sold it rather quickly.. Learning japanese is a big undertaking, and for the most part, it'll take you years and years before a japanese videogame will make much sense. As for games without much japanese required, a person with no japanese knowledge can pretty much understand these by using a kana table and dictionary. anyway these games are few and far between.
Later on, when my japanese was good, I bought another PC-FX and have had more fun with it, but still, in all honesty, there aren't too many reasons to own one. I'd probably sell mine again but I'm afraid I'd end up buying one a third time for some other reason. Just as an aside, while learning japanese, the only part of videogames that helped was their manuals. Japanese takes a lot of effort to learn and in my opinion, playing japanese videogames as a method of learning is just plain awful. You wouldn't enjoy the game and you won't learn much, simply. The Dragon Knight 4 intro is like an hour long.. And it'd probably take a beginner (maybe a year of japanese study) about 3 months of part-time work to translate it, and then most of it wouldn't make sense. Japanese videogames are a lot more fun when you understand Japanese but they won't do much to help you learn.
The turboduo has a lot of great games. I'd probably pick this one.. however, I see most of them going for a lot of money and I might actually pick a newer console (PS2 or something) for all the new features, the support, the new games, etc. Anyway thats my 2 cents.
-RUiNa