Where English is enough
In Playas del Coco, Tamarindo and the other established coastal towns, the businesses that serve foreigners operate in English. Restaurants, real estate, property management, tours, many clinics. You can arrive with nothing and function from day one.
Tamarindo in particular is the most international town on this coast, and you will hear more English than Spanish on the main street. That cuts both ways, and which way it cuts for you is worth thinking about before you choose a town.
What it costs you
Three things, in rising order of importance.
Money. Not through malice, but because the person quoting you a price for a repair, a car or a piece of work is quoting to someone who cannot compare, cannot negotiate and cannot ask a neighbour. The gap is real and it never closes on its own.
Comprehension. The rules, the notices, the reason the road is closed, what the neighbour is actually annoyed about, what the bank clerk needs from you. Living without this is manageable and quietly exhausting.
Friends. This is the big one. Without Spanish your social world is other foreigners, which is a smaller and more transient group than it looks in your first year. People come and go. Costa Rican friendships are the ones still there in year five.
How much is enough
Less than you fear. The useful threshold is not fluency, it is the point where you can open an interaction in Spanish, apologise for your Spanish, and let it continue. That earns you a completely different reception, and it takes months rather than years.
Ten minutes a day before you move, then daily use once you arrive, gets most people there. The move itself is the accelerator, because you are surrounded by it.
If you want the fast version
Live somewhere it is unavoidable. Ten minutes inland from the coast, prices drop and Spanish comes back, and you will learn because you will use it. That is a harder landing for a first year, and it is the single most effective thing anyone does about this.
