If you stick to the San Jose or Jaco area, you can get by forever not knowing one word of Spanish. The thing is, however, once you live here and you're not just on a 4 day whirlwind find 2 a day hot poontangs besides eat, sleep, see sights, shop, walk around and plan your next trip, you slowly start learning just by interacting with Tico's/Tica's on a daily basis.
You quickly learn the basic most useful words, banyo / bathroom, ''?donde esta el banyo?" / where is a bathroom ? directo / ''go straight" to taxi drivers, derecha, izquierda and alto ( right, left, stop). Now you can direct any cab driver anywhere and search for a bathroom....
English and Spanish are sister languages, both born out of Latin and share many words. No matter what your first language is, Spanish is considered the easiest of all languages to learn. Just through curious osmosis you will be picking up words and phrases necessary to just get around, ask about prices, get directions, get food and of course, find a bathroom....
Now since you're retired down here, you don't want your brain focused on eating, sleeping, gambling, drinking and poon tang exclusively. There are many local Tico/Tica schools to teach Americans not just Spanish, but Costa Rico Spanish, and the lessons are cheap by our American standards, plus you can make it a fun challenge by listening to tv and radio, reading Spanish newspapers and chatting with locals, I always find they are friendly and patient and excited to teach, they appreciate that you are interested enough in them to want to learn their language, I don't think I ever was refused explanations and instructions when I asked.
So there you have it, if you're focused and determined, you could be an excellent communicator in about a year, even if you want to "puda vida" it and take your time and learn slowly, you'll be excellent in about 2 years...as long as you are serious....
! No hey problema, amigo, no hey problema !