You may say 'anata ga inakute, totemo sabishii,' written: あなたがいなくてとても寂しい
Nic and Rocco
Anata o ketsujō shimasu
I Really Miss You = Te Extrano Mucho They are not exact words of the translation but that's how you'd say it.
Isa
Watashi wa anata ga inakute sabishii
ビアさん (Bia-san)
Hontou ni.
Xinda
You can say "ਮੈਂ ਕਿਸੇ ਨੂੰ ਬਹੁਤ ਯਾਦ ਕਰਦਾ/ਕਰਦੀ ਹਾਂ" (Main kise nu bahut yaad karda/kardi haan) to express "I really miss you" in Punjabi.
With lots of emotion, say, "You know . . . I really miss you!" It is just normal communications with another human being.
Sugoku oishii
-.- I can speak Japanese and you don't say it any differently really, except for maybe the accent I suppose.