I love slang terms and observing the evolution of language, and the merging of multiple languages into one -- all language evolves and changes if it is used. For that reason, I find phrases like (kids today) "don't even know how to use the English language" pretty obnoxious. If you can successfully communicate an idea to your audience, then you know how to use the language.
One of the good parts of our globalization is the way different languages influence each other creating new idioms and (sometimes) dialects. Furthermore, English is a language which evolved from Latin, German, French, Welsh, whatever the Saxons spoke, etc. and in the U.S. has since been profoundly influenced by the Bantu and Amerindian language families, as well as Spanish.
When I was getting my B.A. (Spanish major, English and History minor), I was constantly chided for destroying the "purity" of each language by using una mezclada de both para claridad y efficiency. That was a demonstration of Spanglish/ españinglés for speed and efficiency.
Sometimes an idea is communicated more precisely or comprehensively with a word from another language, like the different levels of loving (querer and amar) and being (ser and estar). Another example is the German weltenschaung which means world view. It describes a perspective somewhere between paradigm and point-of view.
Then there is slang. Sometimes a slang term is so effective it evolves into an idiomatic expression -- a term or quirk of the language with a meaning that defies literal translation. A good example of this is catching a case. It means to be charged with a crime, but it expresses the arbitrary nature of arrests and sentencing in the U.S. -- like catching a cold.
I was standing too close to that rabid, power-thirsty rookie when he sneezed and now I've caught a case. I go to court tomorrow.
Or...You caught a case? You need to avoid those idirty streets.