There are pros and cons to being polyglots/bilingual. Each language has its own structures and syntaxes that are unique among all people. The pros for being bilingual are: 1) understanding two different groups of people and 2) being metalinguistic (aware of how the language is structured). The cons for being bilingual are: 1) slow cognitive development and 2) sometimes mixing up language structures.
I speak two languages, English and Hmong (green dialect). I have attempted to learn Spanish throughout grade school and although I comprehended the basics I could never get myself to speak fluently without looking up terms and stuttering. It was a lot easier for me to pick up English as a second language because I learned it when I was younger, around the ages of 3 and 4. I did not take Spanish until I was about 13 to 14 years old. Children tend to grasp these understandings a lot quicker than adolescents or adults. I also think that speaking two languages has made me high in metalinguistic.I even at times find myself mixing both English and Hmong words together in my sentences when I communicate with friends and family. The main reason why I do this is because some Hmong words can't be translated to English just as how some English can't be translated to Hmong. This shows how complex languages are within certain groups.
Scott Lilienfeld, 2nd ed. Psychology: From Inquiry to Understanding. p.293