Is there any way to translate words in Safari on iOS and Mac? I am currently reading a few websites in European languages (Spanish, Italian, German and French), and I'd like to be able to quickly translate words that I don't know (not the entire articles, which is why I don't want to use Chrome's translation). I know there's a "define" feature, and it is nice, but it only gives definitions in the same languages as the word of interest, so it is not exactly useful for someone unfamiliar with the language.

So, maybe someone knows a feature or an extension similar to the "define" one, but that would allow me to quickly look up a translation for a word? I'm interested in solutions for both Mac OS X and iOS (latest versions). Maybe there are third-party Italian-, German-, and French-English dictionaries for Apple's "Dictionary" application?
For OS X you can add 3rd party dictionaries.  See

http://m10lmac.blogspot.com/2013/01/adding-dictionaries-to-dictionaryapp.html

Unfortunately it's not possible for iOS yet as far as I know.