Ideally, slang and foreign terms should compose 10% of an average conversation, even where they heavily influence the local culture (e.g. tech stores, skate shops, ranches and so on). On top of that, the nature of these terms is very volatile, which makes them susceptible not only to falling out of use in mere days of their invention, but also weeding out those not in the know and barring them from ever becoming part of the inner circle.
Why, then, should anyone bother to employ something so fickle and exclusionary?
Well, for one, it strengthens the bonds between us and those we speak with, as well as encapsulate concepts that'd take paragraphs to describe (such as Salty, the sports term to describe bitterness over an unfavorable situation, typically losing at something); like wise, using foreign terms can help you when you know the word you wanna use, but don't know it in Japanese yet-which will be quite a bit in your initial experiences with the language and long after. Most importantly, there will always be terms the language doesn't have that everything you've learned will help you express. Like what, exactly?
Names are the area they'll make an immediate impact, specifically if they don't fall in line with traditional Japanese phonetics; same goes for terms tied to certain lifestyles and ways of doing things, terms that come with their own history and weight, terms that tell the listener the speaker thinks they know enough about the culture behind it to embrace all that and use it in a conversation. The best way to discover whether you have enough of that to use terms like Grinding or Mixing is the same way you can get all you've learned-as well as stuff I might've overlooked-down pat: chattin' with folks; that single act can teach you more about slang and how to use it than anything or anyone-myself included-could ever dream of doing.
Books are static, blogs are static, people aren't and damn sure don't talk that way. As I see it, conversation is a living ocean with its own ebb and flow, never the same way twice and always demanding the focus of those riding it, lest it swallow them whole (though you can always try again, unlike actual ocean travel). Don't think this the only sure shot method to learning it, but since auditory memory sucks when not in use, you'll wanna reinforce what it gives you by pairing it with other methods-taking cell shots of where you parked your ride, for example, decreases the chance you'll need to comb the lot just to find it again.
It's not guaranteed to make it stick(as guarantees are as real as tasty, hearty and cheap vegan cuisine), but all you've picked up will steady that initial voyage and give you the tools to plot your language learning course, wherever the destination might end up being. Safe travels, and may this info make your journey more like sailing these oceans instead of swimming across them.
Special thanks go out to Tomo Akiyama, who's assist during the planning phase played a major role in setting this thing off proper. Without it, this would've been a research nightmare. Greatly appreciated, Akiyama-san(orz).