Say I'm learning Spanish. I'm learning the Latin American dialect so I'm familiar with slang idioms in the dialect. On the other hand, I haven't had much contact with speakers of "Spain" Spanish. Do I need to surround myself with these speakers as well to learn their vernacular slang?
Similarly, if I'm learning English, do I need to talk to British, Australian, as well as, American speakers to learn their individual?
It's always good to get a wide variety of input, and this isn't just limited to dialect. It's good to be exposed to different registers and speech genres as well. By getting exposure to all of these, you'll encounter more kinds of words and phrases, and hopefully you'll start to understand when certain vocabulary is appropriate and when it is inappropriate.
But it's not necessary. If you're going to spend most of your time in one area where one language variety is dominant, then you can learn most of what you need while living in that community. It's best to focus your time on the aspects of the language you'll use most, especially when first learning the language. So if you're learning Spanish in Latin America, I don't think you have to worry that you're not getting enough "Spain" Spanish. If you can watch some Spanish movies, great; but take advantage of the environment you're in, and acquire as much from the locals as you can.
For comprehensive fluency in a language, you should be very familiar with the all aspects of a language including slang and informal, colloquial use as well as literary and formal register. Though you may not be able to use all of them (eg. It would sound very weird to us if President Obama started speaking NY Bronx-ian English).
Even for native speakers, it is common for people to only ever speak with their regional dialect and vocabulary. This helps distinguish between different geographical regions of a language. For instance, if I asked you what you would call a shade structure that is usually found in a park or outside somewhere, what would you call it? If you said ramada, you are most likely from southern Arizona. If you said gazebo, you are most likely from the East Coast. If you said pavilion, then you are from somewhere else.
The necessity of knowing all slang is not necessarily useful or even helpful. Learning a language is (or should be) primarily for communicating with another person or group of people. Some slang can hinder that communication if used with the wrong group. For example, If I asked you to go out onto the veranda and over to the ramada to grab a tooney from under the chesterfield, would you understand what I was even saying? I used "slang" (colloquialisms) from Arizona, England, and Canada in my sentence, but the meaning is muddled.
Overall, it is good to learn the different colloquialisms from around the world as long as you are consistent with your own usage not to mix them. Doing so might adversely affect effective communication.
