According to Kansas University psychologist Michael Vivevitch, bilinguals do not confuse the languages they speak mostly because each language has a unique sound neighboring system. In other words, just like human DNA where A, T, G, C have an identifying order, the sounds that follow each other in Spanish and English are different. Thus, bilinguals can easily distinguish which word to use or which language they are spoken to.

This is What Michael Vivevitch wrote

“A corpus analysis of phonological word-forms shows that English words have few phonological neighbors that are Spanish
words. Concomitantly, Spanish words have few phonological neighbors that are English words. These observations appear to
undermine certain accounts of bilingual language processing, and have significant implications for the processing and
representation of word-forms in bilinguals.”

“The results of the present corpus analysis show, in several ways, that words in a foreign language do not “invade” the lexical neighborhoods of another language. That is, for the two languages examined here, there are few words in one language that are phonologically similar to words in the other language. This simple observation raises a number of important and fundamental questions about lexical retrieval and language processing in the bilingual. First, the minimal amount of phonological overlap between the two languages essentially creates two separate – or perhaps, easily separable – lexica. (Note that other low level phonological information might further contribute to the separation of languages; see e.g., Ju & Luce,2004.) The D E FAC TO separation between languages based on their phonological characteristics raises a question about the need for explicit representational schemes, such as language tags (Green, 1998) or language nodes (Dijkstra & van Heuven, 1998), or other cognitive mechanisms (e.g., Bialystok, 2010) designed to keep the word-forms of one language separate from the word forms of another language. If one considers the small number of words that might benefit from such measures, these approaches to language processing seem cognitively and computationally expensive (and seem increasingly expensive for the individual who knows a third, or fourth, etc. language).”
 

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