I have been told by most people who hear me speak Spanish (my mother tongue is English) that I am fluent in it. That alone should be enough for me to call myself bilingual. However, I have encountered numerous situations over the years where I felt completely lost in Spanish. So, if I am so fluent, why would this happen to me? The answer is a simple but often overlooked one. Bilinguals usually acquire and use their second ( or third or fourth) language for specific purposes and when they get outside of this specific linguistic domain, their experience (and the language which represents that experience) is limited.
Outside of our mother tongue, we generally spend most of our time speaking our new language in a limited number of areas. A number of years ago, I found myself in an academic setting in Puerto Rico where I had to teach courses in Anthropology in Spanish. You would think that since I had already been speaking Spanish for over a decade and had a Ph.D in Anthropology, the transition would be an easy one.
Well, I can assure you that it was not. My basic lecture skills were sufficient, but my professional training -- all in English -- left me lacking in vocabulary to describe the technical vocabulary of my discipline. It was like having to learn a new language. I knew my discipline but I did not know it in Spanish. My Spanish skills had been developed in normal everyday and familial encounters, but was extremely lacking in the terminology of anthropology. If I had not found a Spanish translation of the textbook I had previously used when teaching in English, it would really have been a difficult transition.
This lack of language use in different life domains can have a tremendous effect on our ability to communicate in our second language. Each domain has its own particular vocabulary and unless you have experienced that domain previously you may -- and probably will -- experience some discomfort in having to speak the language. Your degree of fluency is much greater in the domains in which you have most frequently used the language. For a person who is exposed to several languages, the language which is most used in a particular area of life will be the one used when discussing that area of life. That explains why a person who appears to be completely bilingual will frequently switch over to the dominant language which they have used most in that area of their life.
It has been said that you are totally bilingual when you dream in both languages. Even that is questionable. You may be completely bilingual in certain domains (and in these you may dream in both languages), but in other areas of life (particularly technical ones), you will always find your abilities lacking until you are forced by circumstances to learn the vocabulary and related cultural components of that linguistic domain. So, how bilingual are YOU?