Quote from http://www.nintendoworldreport.com Just Dance Wii had content from the first 2 games. Just Dance Wii 2 is a modified version of the third game in which "Nintendo updates the game to be more appropriate to Japanese culture."
JDWii2 song list from amazon.co.jp (Google translator)
Music / artists (Alphabetical)
INDEPENDENT WOMAN / AI
/ Wink lonely tropical fish
Rising Sun / EXILE
Lovers Again / EXILE
Kaoru and Mori Yuki Mori Mar Mar! /, Mook occasionally.
Summer GO GO! / KARA
Roller coaster Love / KARA
Performance formulations formulations carry performance PON PON PON /
Mickey Mouse March (Family Parapara Version) / Cutie ★ Mommy
Shonan no Kaze / water lily flower
Body & Soul / SPEED
Takken grace Joyful (cover) /
We can't stop the music / DA PUMP
Bo Peep Bo Peep / T-ARA
Knight raised up ♂ ♂ EVERY ☆ (Night) / DJ OZMA
I'm your man/2PM
We Can Fly / Happiness
Aya Matsuura Holiday / Truly Yeah!
Ride on time / MAX
POP STAR (cover) / Ms.OOJA
Music / artists (by ABC)
Girlfriend / Avril Lavigne
Beautiful Liar (cover) / Countdown Mix Masters
California Gurls / Katy Perry featuring Snoop Dogg
TiK ToK / Ke $ ha
Mamasita / Latino Sunset
Party Rock Anthem / LMFAO feat. Lauren Bennett & GoonRock
Lollipop / MIKA
Promiscuous / Nelly Furtado feat. Timbaland
Only Girl (In the World) / Rihanna
Spectronizer / Sentai Express
Gonna Make You Sweat (Everybody Dance Now) (cover) / Sweat Invaders
Dynamite / Taio Cruz
Pump It / The Black Eyed Peas
Baby One More Time (cover) / The Girly Team
She's Got Me Dancing / Tommy Sparks
I can't speak for the person you insulted, but for me the superiority of the Japanese Just Dance games has nothing to do with the fact that half the songs are Japanese (and Korean). I actually only genuinely like a few of the Japanese songs on these, while collectively there are more songs on the English versions that I like.
No, it's not the music that makes them so much better. It's the dances. If you're any good at these games, you can score over 10K easily on the most complicated Just Dance 2/3 songs after just a few tries. Not so for a lot of songs on the Japanese versions. Download Just Dance Wii and play Tohoshinki's "Why (Keep Your Head Down)" and tell me that isn't some ridiculously hard shit. Or just look up the video on Youtube, because the Just Dance choreography is identical. The EXILE song on that version is pretty tough too, and overall the choreography on EVERY song is more complicated than in the US counterpart (and it's easy to compare, since half the US songs are included in the JP version).
I haven't played enough of Just Dance Wii 2 to determine the hardest song, but I'm betting it's some crazy K-Pop boy band nonsense. Play through a few JP/Korean songs then play a US song and see how easy-mode our Just Dance games are. Especially if you expect to do the footwork, which isn't scored, but if you're a dancing badass you'll do it anyway.
Oh, and this version includes a training mode that lets you rewind and retry your moves over and over again until you master them. It's extremely helpful if not necessary on some of the more complicated dances.