I’d like to say I’m fluent in Spanish. I guess it depends on how you define fluent. I can make myself understood. I can usually understand. I couldn’t explain the inner workings of a jet engine to you, but I couldn’t do that in English, either. I’m missing a lot of specialized vocabulary. I would struggle with legal jargon, for example. Over the years, I have gotten lazy and relied on Google Translate a bit more than I probably should. But, yeah, I can read books and watch movies and chit chat, for the most part.
But one thing I have always studiously avoided was learning slang. First of all, there are a lot of Spanish speaking countries out there, and each has its own slang. Second, slang, by it’s very definition, is time sensitive, and there’s nothing cheesier than someone using slang that’s outmoded, Daddy-o. I have no access to that cultural calendar that seems so instinctual in one’s first language, at least if you’re in with the in crowd.
Also, a lot of slang is based on cultural references that I’m not privy to. And slang often has its place in certain groups, but not in others. For example, there are things you’d say to your friends that you’d never say to grandma. Determining the appropriate audience for slang is a challenge in English. I doubt I’d be able to cope in a second language.
So my advice to anyone learning a second language is to avoid slang at all costs. It’s just too risky. You could offend someone without intending to. You may want to look cool, but you have an even better chance of looking like a fool. It never hurts to proceed with caution.