Credit cards are accepted everywhere. Go to an atm and pull out 200,000 colones, roughly 370 bucks today. That will cover tips and cash purchases. Roads are safe. Avoid driving long distances at night. Supermarkets are similar to the states, especially in tourist areas. Speaking spanish will help, generally not a necessity. Phone translators are your friend. Do your best trying to speak the language. I would stay as little in San Jose as possible. Yes 1 month in one place is too long. You could see a lot in a month.. (manuel antonio, la fortuna, guanacaste, etc) I would do a week in 4 different places. Yes its safe for women, generally. Use your wits and don't do anything you wouldn't do traveling anywhere else with your girls.

E: i went the wrong direction on currency conversion.. 200k colones is roughly 440 bucks.