The language predominantly spoken is Spanish. It's fairly easy to pick up, and your efforts to speak it will be graciously received. If your Spanish breaks down, you should have no trouble finding people who speak English in major cities. Many guided tours offered at museums and historic sites are in Spanish; ask what language will be spoken before signing up.
Phrases
Yes - Si
No - No
Hello - Hola
Goodbye - Adios
Please - Por favor
Thank you - Gracias
My name is ... - Me llamo ...
How are you? - Que tal estas?
I'm very well - Estoy muy bien
I feel ill - Me siento mal
How much does it cost? - Cuanto es?
Do you speak English? - Habla ingles?
I don't understand - No entiendo
Where is ...? - Donde esta ...?
Entrance - Entrada
Exit - Salida
Danger - Peligro
Open - Abierto
Closed - Cerrado
Toilets - Servicios
Doctor - Medico
Hotel - Hotel
Restaurant - Restaurante
Beer - Cerveza
Wine - Vino
Menu - Carta
Today - Hoy
Tomorrow - Manana
Monday - Lunes
Tuesday - Martes
Wednesday - Miercoles
Thursday - Jueves
Friday - Viernes
Saturday - Sabado
Sunday - Domingo
One - Un / uno / una
Two - Dos / dues
Three - Tres
Four - Cuatro
Five - Cinco
Six - Seis
Seven - Siete
Eight - Ocho
Nine - Nieve
Ten - Diez
Twenty - Veinte
Thirty - Treinta
Forty - Cuarenta
Fifty - Cincuenta
Sixty - Sesenta
Seventy - Setenta
Eighty - Ochenta
Ninety - Noventa
One Hundred - Cien
One Thousand - Mil