Not all TCKs are multi-lingual, but a large portion of us are. We know how this benefits us in every day life, if we use the languages we speak. But do you know how being multi-lingual affects us beyond simply being able to communicate with more people?
A lot of TCKs I know claim that they are not, in fact, multi- or bi-lingual because they "are not fluent," "can't read or write well," or even "have a strong accent" in their second language. However, this is not necessarily true: for the purposes of study, being "bilingual" is a broad ability. Pretty much any ability in a second language is grounds for at least some of the benefits of being bilingual. (http://www.francoisgrosjean.ch/myths_en.html)
Now, let's talk specifics: first up, being bilingual improves our working memory and executive function.
Working memory is what allows us to store and retrieve facts from our brains. It is the memory that actually holds the information that we are currently working on. In studies, bilingual children were faster than mono-lingual children in performing tasks that required them to hold and manipulate more information. This may be evidence that working memory is a little more efficient, better managed, and able to process more information faster if you are bilingual. (Morales & Bialystok, 2013)
Executive function is the ability to control other tasks in our brain: the control panel, basically. It is what helps us switch between tasks, decide how to manipulate the information in working memory, etc. While bilingual children in studies performed better on all working memory tasks, their advantage was even more pronounced in taks requiring more executive function. This indicates that perhaps the ability to switch between languages and determine how to use the vocabulary they store improves bilingual children's ability in executive function. (Morales & Bialystok, 2013)
Perhaps because of these improvements to general brain function -- including physical changes in the brain, like denser gray matter (Kovelman, Baker, & Petitto, 2008) bilingual people have an advantage in diseases like dementia. It appears that being bilingual can stave off dementia, protecting the brain from the negative effects of the disease. (King, 2013)
So, basically: Being bi- or multi-lingual, which many TCKs are to at least some extent provides us many advantages. It gives us better working memory, better control over our brain function, physical brain differences such as denser gray matter, and even protects us (to some extent) from dementia. So if you are a TCK that speaks more than one language: yay you! Don't downplay or discount that amazing part of your experience!
Resources:
Morales, J., Calvo, A., & Bialystok, E. (2013). Working Memory Development in Monolingual and Bilingual Children. Journal of Experimental Child Psychology, 114(2), 187–202. http://doi.org/10.1016/j.jecp.2012.09.002
Kovelman, I., Baker, S. A., & Petitto, L.-A. (2008). Bilingual and Monolingual Brains Compared: A Functional Magnetic Resonance Imaging Investigation of Syntactic Processing and a Possible “Neural Signature” of Bilingualism.Journal of Cognitive Neuroscience, 20(1), 153–169. http://doi.org/10.1162/jocn.2008.20011
http://www.francoisgrosjean.ch/myths_en.html
http://ed.ted.com/lessons/how-speaking-multiple-languages-benefits-the-brain-mia-nacamulli#watch
King, B. (2013). New Study Shows Brain Benefits of Bilingualism. NPR Science. http://www.npr.org/sections/13.7/2013/11/14/244813470/new-study-shows-brain-benefits-of-bilingualism