'je te kif tro grav' -what does it mean?

I'm not French, but I'll do my best!

Je te kiff tro grav = Je te kiffe trop grave. Je kiffe is a slang way of saying "I like", or maybe "I love", so literally it would be something like "I like you too badly". I think if I were translating it I might use something like "I think ur reeli hot", but it's so slangy that it's quite hard to find an exact translation.

for want of a better phrase, but this is the most appropriate and symmetrical: "I dig you"

je te kiffe (I fancy you, I really like you or 'dig')
trop grave - too serious ;(so much)

kiffer = to love

trop = too much; young people use that constantly, a bit like "so" in English, e.g. this conversation is SO over.

grave = serious, but again, it's a word young people use to accentuate what they are meaning.

The subtitles on my DVD said in English:

T bel (tu es belle) = UR awesome
jte kiff tro grav (je t'aime beaucoup) = I'm 2 hot 4U

The translation is absolutely correct!

it means "I dig you so bad"
typical street language. It's a tad crude, so in no way is it the equivalent of "I love you too much" or whatever.
Infact it's not romantic at all, which is why the girl thinks he's a freak in the beginning

I'm french and live in france, so I think that's the best translation