Since this post goes out a couple days before St. Valentine's Day, here are some translations if you're not familiar with them.

 

Here's the literal translation of St. Valentine's Day:

 

St. Valentine's Day
día de San Valentín
dEE-ah deh sahn vah-lehn-tEEn

 

In a lot of places the literal translation isn't used as much as
one of the following:

 

Day of Love and Friendship
día del amor y la amistad
dEE-ah dehl ah-mohr ee lah ah-mees-tahd

 

or

 

Day of Lovers
día de los enamorados
dEE-ah deh lohs eh-nah-moh-rah-dohs

 

Asking someone to be your "valentine" isn't really used but here are a couple literal translations. These would probably work better speaking Spanish in the US rather than Latin America or Spain:

 

Do you want to be my valentine?
¿Quieres ser mi Valentín?
¿kee-ehr-ehs sehr mee vah-lehn-tEEn?

 

or

 

Do you want to be my valentine?
¿Quieres ser mi pareja de San Valentín?
¿kee-ehr-ehs sehr mee pah-reh-hah deh sahn vah-lehn-tEEn?

 

In Latin America or Spain rather than saying "valentine" it would be better to say something like this:

 

On St. Valentine's Day do you want to be my boyfriend/girlfriend?
¿En el Día de los Enamorados, quieres ser mi novio/a?
¿ehn ehl dEE-ah deh lohs ehn-ahm-oh-rah-dosh kee-ehr-ehs sehr mee noh-vee-oh/ah?

 

Can't forget Cupid:

 

Cupid
Cupido
koo-pee-doh