It is the age-old story. You studied a language in school and then tried out your awesome language skills on a native speaker. They rapidly fire back an answer to whatever question or greeting you used and start babbling on unintelligible strings of sounds that hit your ears with the speed of a machine gun. You then think to yourself that you were not at all prepared by your classes for this conversation. Now you are wondering that if a classroom is not where it is at, where should you learn a language? The answer may surprise you.
When it comes to fluency, the best place to learn a language is always going to be with native speakers in an immersive environment. However, learning exclusively with native speakers on the street may not always be the best way for everyone. Say your job involves working with native French speakers from Quebec. You learn your French exclusively by talking with your colleagues and get no formal training. Although you are able to become fluent in spoken French this way, there are some disadvantages to not studying French formally. The main downside is the fact that you will not have learned how to read or write French. If you are learning French for business purposes, knowing written French will be essential for your job.
Another disadvantage would be that the French you are learning will not be standard French. You will likely have learned much slang in your conversations. Although this is quite useful, it can become an impediment when it comes to speaking with French speakers from other French-speaking countries. For this reason, learning a more formal, standard French is likely more ideal than learning on the street. Taking classes will also help you learn standard pronunciation as well.
When faced with a fast-paced conversation that you are not able to understand, the reason is not necessarily that you learned the language in a classroom versus with native speakers. The problem is more likely that the class was not formatted ideally. The ideal language class has a highly trained professional who is well-versed in the art of teaching languages and in the language learning process. The best language classes are also focused on "real language." Real language is natural language that would be used in real-world contexts. This focus will help you avoid that awkward moment of not understanding a single thing coming out of a native speaker's mouth.
The ideal language classroom is focused on real language which includes slang and street language used in the context of conversation. Ideal classrooms should be student-focused rather than lecture-focused. Students will be seen conversing together and communicating via written and spoken means with fellow classmates as well as with native speakers of the language they are learning. After you have been taught the standard language, you can move your focus to learning slang and vocabulary specific to the country of the native speakers you speak with the most. By learning the standard language, you will be understood by almost all native speakers of the language you are learning, and you can work on specializing your dialect if you so desire.