Well, in my language there aren't many special words we say, usually just "I love you" works fine. In Japanese there is the saying "Tsuki ga, Kirei desu ne?" though, which means "Isn't the Moon beautiful?" which has the meaning of "I love you." I forgot the exact reasoning behind it, but I think it was based on some old old saying that women shouldn't be direct and could communicate their feelings by being very subtle, and then they gave the example of how a woman saying that the moon is beautiful meant that they loved someone... Or something like that. When you think of it in a historical context, it's actually a bad phrase due to its implications (or at least I think so, tbh, I don't know much more about this phrase's origins because I only googled a bit about it, so maybe Japanese historians and linguistics don't feel like that), but... Isn't it kinda beautiful to tell someone you love them by just talking about the moon? I think it feels romantic.
I don't really know what lovers say , but for family we say: (احبك مقد السما والاغض) ( *احبك ايهلقد * مع فتح الايدين) (اموت عليك ) that in our accent ، in classical Arabic its: ( احبك بقدار السماء والارض ) (*احبك بهذا المقدار *مع فتح الذراعين)
The most common thing people in my country would say is: ฉันรักคุณ (Chan ruk kun) which translates to the iconic "I love you". A little bit of explanation here if you're interested: in Thai, when a person refers to themselves we usually do so by differentiating between gender. "ผม" for men and "หนู" and various others for women, but they're all equivalent to the English " I ". The sentence above "ฉันรักคุณ" simply makes use of a gender neutral pronoun, "ฉัน" (chan). Anyway, I'm gonna be reading some more fluffy romance in preparation for this coming Valentine's day(mainly because I don't have a partner to celebrate with) and maybe I'll buy some chocolate while I'm at it, since those always go on discount at this timing of the year. Well, chances are I probably wouldn't be doing any of that, since I'm gonna be stuck at military camp for the whole week.