This aria is from Camille Saint-Saëns’ opera “Samson et Dalilah”. In it, Delilah seduces Samsom in order to get the secret of his strength.  
“Rarity Investigates!” is my favorite episode not just of the whole fifth season of MLP, but my favorite episode of the whole show. And my favorite scene is where she seduces the castle guards for information on their whereabouts the night the fake letter was sent to Spitfire. So I decided to put the scene and the aria together.
 
The translation of the lyrics are as follows:
 
My heart opens to your voice  
Like the flowers open  
To the kisses of the dawn!  
But, oh my beloved,  
To better dry my tears,  
Let your voice speak again!  
Tell me that you are returning  
To Delilah forever!  
Repeat to my tenderness  
The promises of old times,  
Those promises that I loved!  
Ah! respond to my tenderness!  
Fill me with ecstasy!