I have played every version of Gran Turismo since the second game back on PlayStation, in Japanese form and have never come across any problems. I don't speak a word of Japanese, and can only read a few words of Katakana (namely save, load, yes, no, controller, replay, exit cancel, etc).
It will be fine in Japanese form, and there will be loads of help on here and elsewhere if you get stuck.
Right now, I'd like to know more about the HDTV options. 1080i ingame sounds a bit much to be honest, as in a E3 IGN interview the producer said they couldn't even do 480p as the performance hit on the hardware was too great. Sounds interesting though!
Also, the packaging is of interest. The Japanese versions (early ones) of GT3 came with an extra book - I wonder if GT4 is the same? And the pic on the linnk on the first page here, it looks like the game comes in a outer box, like a cardboard sleeve too.