When I was younger, I always looking at the language merely as a mean of communications. So, as long as two persons could understand each other it does not matter, which language or mix-language they speak. Back then, I did not pay much attention to the fact that coherence in language is important for expressing ideas by brain. Every new word is stored with a spectrum, a wave packet, of some stories, objects, and cultures attached to it. By mixing different languages these spectrum could easily overlap and lead to blocking efficient communication. Since then a single concept comes with two output values, hence, it is not a function anymore! so it does not act as a function any more! To be more explicit, consider the word science in English and علم in Persian. They refer to seemingly same subject but with deep differences. Therefore, abusing and misusing them would block the efficient communication.
As much as learning other languages is important we should remember that the inner self of us probably talk to us in our mother language. Probably, the ideas and interconnections between different memories and knowledge stored in our brain is in our mother tongue, that is, the coding and decoding in our brain is probably in our first language in the very fundamental level. We may speak or write well in other languages but it does not mean that brain is also functioning in those languages. By mix-using different languages, however, there is a danger for a brain to loose the connection between the inner-self and itself in communicating ideas.
We should also remember that every word comes with a culture decoded deep in the word. So, learning a set of new word opens up a window toward a new culture. However, as the newly imported plants may ruin the life of native habitat, newly imported word could also destroy the usage and spreading of the original language. As there is a set of laws enforced heavily for import-export of everything, we should do the same for importing new words from other languages into our language. In this way, not only we save our language, we also let new words, new ideas, and new culture to be developed within our language which finally led to new movies, new science, and new discoveries.
I encourage every one to read this nice article by Prof. Reza Mansouri, about “how and why to develop a scientific Persian language” which is in Persian.
PS: Thanks to my wife for sending this article to me which triggered this post. I also like to thank my Friend Payman, for originally posting this link on facebook, and also for a good discussions on this subject before.