ترجمه - ژاپنی-انگلیسی - kimi no egao wa, sekai de ichiban kirei ...

The meaning is perfect, but in North American English, I think we would add the word "one":
Your smile is the most beautiful one in the world.
or you could say
You have the most beautiful smile in the world.

But the meaning is perfect, anyway!

Thank you, I think the better translation is your second sentence: "You have the most beautiful smile in the world." I'll use this sentence.