Whatevs, comes from what ever and Lang Lang is just a random word used to make the whole statement seem more fuller and have more impact by repeating a word.
Adam: Your hair looks ranga (ginger)
Josh: Whatevs Lang-Lang
Phrase uttered by the character, Gail the Snail of the TV show It's Always Sunny In Philadelphia
Gail: Hey guys, you wanna come to my car and take a puff? I got some medical marijuana.
Dee: We’re at your dad’s funeral, Gail.
Gail: Whatevs, I'm over it, plus, if we all show up super high at the reception, everyone would be like,