To echo the above, it's super import friendly. The menus are in english, there's a ton of songs (none of which I'd heard of, but many of which I now like
),
and there's a budget re-release that's easy to get ahold of, so if you're remotely interested I'd say pick it up. It's well worth it.
You really don't need any Japanese to play. Even the mission/challenge mode objectives are pretty obvious to work out.
To give an example, the Step-Up mode has mission objectives denoted on the side here. There's a "Random" icon that gives you random buttons to press instead of the standard ones, the "Hide" icon hides the notes once they get near the hit zone. The Color icon means you have to hit the Color Notes with specific buttons.
For a "Color" Mission, when you go to select the mission the color icon will have a button press in there somewhere - so in this screenshot it's Triangle. That means you have to hit all the color notes for this mission with the Triangle button. (And the Rank bit means you have to get Rank A or higher).
Congratulations. You now know how to play IA/VT.
The only other thing is to make sure your game is saving, because the message on start up is all Japanese. To check, you can go to the options menu which looks like this. If the fourth option is "On" then your game will save automatically.