In Japanese you say 「お久しぶりです。(O-hi-sa-si-bu-ri-de-su)」 if you'd like to be polite.To be more friendly, it would be better to say 「久しぶり。(Hi-sa-si-bu-ri)」
"Long time no see" is just an expression in english. To say "I have not seen you in a long time" in french would be said as "Je ne vous ai/t'ai pas vu depuis un long moment." or maybe even better:...