First of all, there are exceptions to every rule. There are no "always" and "nevers".
I have been to a lot of countries, and USA people have a worldwide reputation that is affected by many things. There are numerous sterotypes that have to be overcome. Lanuguage can be a help. If you know the natives' language, it helps a lot in making you seem sincere, although that is just a very small beginning. I used to speak a fair amount of Russian, which is pretty unusual for Americans, and it did not get me very far -- I would say that I am not the most personable person, so that has a negative effect on me even if I speak the language.
Language is a big plus, and a lot of guys have had a great experience speaking it. It clears up misunderstandings and shows some interest on your part to be serious in your dealings. However, worldwide, I would estimate in my experience that 90% of USA guys who speak the native language are seen as plastic americans who happen to speak the language. Every visit to CR I get girls from SL, HDR bartenders, normal girls that I know, etc. come up to me after speaking with a guy from the USA in spanish, rolling their eyes and saying "glad that jerk is gone." I'm thinking, funny, you were acting like you were in love with him 5 minutes ago. In short, language is not a magic pill (believe me, i've tried and i know), but it is a valuable tool if you're otherwise being genuine.
Chances are, you have already been stereotyped by a girl before the first word is spoken, as:
1. The ugly American. Flashes cash, gaudy jewelry, bitches about how bad things are outside the USA and constantly exclaims how awesome the USA is. Rude, obnoxious.
2. The rich and powerful. All people from the USA are rich. All guys are Bill Gates -- not necessarily good looking or suave, but rich and powerful -- sorry Bill

3. The superstar. You're Brad Pitt in looks, and you're from the USA. It doesn't matter who you are inside, you're a primo catch and she'll do anything to get you.
There are a lot of good girls in CR (and in every other country). But even those require a lot of time to break down to the point where the sterotypes are gone and she realizes that you're a real person. If you think there's bigotry in the USA, you ain't seen nothing yet. In some places like far eastern europe, bigotry is openly accepted and even endorsed. That can be a hard nut to crack.
SR
I was in CR for just a short trip and I found the cross section of women to be basically the same as throughout Latin America, friendly and willing to talk to you. People return smiles... its a nice change from most parts of the USA. My game plan has always been to practice my language skills as much as possible with whoever I run into. The hottest girl I ever ended up spending time with was a Mexican who had been watching me talk to an old man in a bus station. She ended up following me and arranging an "accidental" meeting. Only later did she admit to me that she was interested in me from watching me talk to the old man. My point being, just be real, be yourself, learn the language, engage the locals, and with time, some amazing things will happen to you if you open yourself to them. I love traveling in Asia and Latina America. Pura Vida.