I don't know what to say

I don't know what to say, Mistress.

Je ne sais pas quoi dire, Maîtresse.

— Agora (2009)

I don't know what to say to that.

Je ne sais pas quoi répondre à ça.

I don't know what to say about that.

Je ne sais pas quoi dire à ce sujet.

Now you're here and I don't know what to say.

Maintenant que tu es là, je ne sais pas quoi dire.

— Moscow Does Not Believe in Tears (1980)

I, I don't know what to say.

Je, je ne sais pas quoi dire.

— Ratatouille (2007)

I don't know what to say.

Je ne sais pas quoi dire.

— Mother Night (1996)

This is my message, but actually, I don't know what to say.

C’est mon message, mais en fait, je ne sais pas quoi dire.

— Y Tu Mamá También (2001)

I'm sorry, I don't know what to say.

Je suis désolé(e), je ne sais pas quoi dire.

I don't know what to say to make you feel better.

Je ne sais pas quoi dire pour te remonter le moral.