One thing to remember is that while a hard-copy dictionary of Spanish slang is great, you've got to keep in mind that slang is one of those things that doesn't remain static for very long.
Think of all the English slang that changes constantly. People don't want to use other people's slang; they want to create their own. And when people in a position to have influence over others (musicians, actors, etc.) decide to replace an existing slang term with another, there isn't a committee out there deciding if it's OK or not.
And with this fast-paced age of social media, this change is even faster.
It's the same in Spanish, which brings me back to what I was going to explain. A dictionary of Spanish slang you find in your local bookstore or on Amazon is great, but it can become outdated rather quickly. Slang changes constantly. Books take awhile to be written, published, distributed, bought, read, updated, republished, etc.
And this can be a real pain if you are in the business of translating Spanish slang or doing transcription work where you have to know the different types of Spanish slang used in the various regions of Latin America.
Learning Spanish Slang
If it's critical that you know up-to-date translations of Spanish slang words, there are multiple ways that you can best understand Spanish slang. Probably the best advice I can give you to learn Spanish slang is to make friends with someone from the country you're interested in and have them teach you some slang. However, this is not always very convenient. For one thing, learning the slang from one region is good, but if you want access to slang from different parts of the region, this will be difficult to get from just one person.
However, there's another way you can have access to more slang in the region. And that would be the Internet. Now, I know I said before that the Internet is not regulated. Nobody checks to make sure that "facts" that people post are in fact true. You can find all sorts of truth and fiction, but luckily, when it comes to slang, there is more correct stuff out there than blatantly incorrect.
However, while that may be a drawback of the information posted on the Internet, there is a plus with that. It's that people can update their information at a blinding speed. There is no need to wait for a publisher, distribution, or anything else that a traditional author of a dictionary of Spanish slang is forced to wait for.
This is beneficial when it comes to Spanish slang translation because people can post the slang from their region online and you can access it without too much difficulty, all without the need to rely on a dictionary of Spanish slang to get the information you need.
In order to help you have the resources you need to translate Spanish slang, I've put together a list here of some links to online dictionaries of Spanish slang that I think are helpful. These are just a couple of the many slang dictionaries on the Internet, and granted, there are many more. If you do find any others that you think would be a good addition to this small list, let me know so that other Spanish translators can have access to these resources as well.
Jergas de Habla Hispana
Probably the best site I've found for lists of regionalisms. There are links to the Spanish-speaking countries with their particular slang terms. I highly recommend this site. It's a good one to bookmark so that you can easily access the words you need when you come across them in your work.
The Alternative Spanish Dictionary
This is an online resource that is attempting to build slang translations of the world's languages. There aren't that many terms but you can either look at the definitions online or download a PDF version. I've also posted a review of the dictionary.
This online dictionary is specific to the Canary Islands. I personally haven't come across many translation jobs that require me to know Canary Islands slang terms, but at least I know that this resource is here in case I ever need it.