The language purists among you may not want to read further as you might be scandalized. I am proposing that for those of us who are bilingual, speaking in both languages within the same conversation is not only acceptable but also may be the most optimal way to communicate.
On the phone with my cousin the other day, we found ourselves speaking at different times in Spanish; at other times, in English. We would complete two or three sentences in one language and then follow with two or three in the other. Only after shifting to the other language would I suddenly become aware of the shift. The transition had been that seamless and unconscious.
It was a fun conversation. It was as if my cousin and I shared a private code which freed us to be natural with each other. We didn’t have to confine ourselves to a given language box. Indeed, one of the reasons speaking that way is so rewarding is that it is the only time I can reflect fully in my speech my specific life story. I came to the US from Puerto Rico at a very young age, after which I spoke only Spanish at home and in church, while at school I only spoke English. The two tracks remained essentially parallel, and to a large extent, except in conversations like the one with my cousin, they remain so today.
I am not proposing that we stop honoring the syntax of each language when in a monolingual setting. I believe in mastering the grammar and vocabulary of each language, and it is only polite to be place appropriate. Though I am sometimes guilty of this, I also try to avoid a language shift within the same sentence. However, when two people fluent in the same languages are conversing, why not take advantage of the greater supply of vocabulary and grammatical structures available?
Literature will inevitably reflect this. One of the things I found appealing about the Pulitzer-Prize-winning The Brief and Wondrous Life of Oscar Wao by Junot Díaz was his seamless incorporation of different languages and styles of speech: colloquial versus learned diction, English versus Spanish, science fiction/comic book language versus regular English. Díaz seemed to recognize that language can no longer be defined by the classroom. It is a lived language. In our increasingly culturally fungible world, this will likely result in more variety and freedom in our modes of expression. At least I hope so.