I played some 30 hours worth of actual playtime (70 or so in reality) of DQ7 (3DS version). I'm not really sure about the difficulty, I've been playing (well, stopped some 2 months ago I guess) it basically since I started reading (only a few minutes/an hour per day though).
I guess the grammar is fairly easy, though I must have mined literally thousands of words from it so there's a ton of vocabulary to be learned. The convenient thing is it has furigana instead of being kana only or kanji with no furigana.
What's off-putting from the dialogue is that it's very stiff and cliche.
To be honest DQ7 bored me to death, I started DQ8 and it's looking better (it's also got some actual difficulty for fuck's sake). But I've had more fun playing other things.
If you wanna play with kana only and have fun at the same time MOTHER 2 is probably better, I haven't played it in japanese yet though.