The poet-philospher Johann Wolfgang von Goethe observed, "To acquire a second language is to acquire a second soul." For, as George Orwell contended, "Language does not reflect a culture; language is culture." It is a curious fact, moreover, that elderly people who have emigrated to America as youths often revert to their native language in their declining years and are unable to comprehend English even after having spoken it for decades.
The nativist theory held by Naom Chomsky states that the human brain has a built-in language acquisition device, or LAD, that extracts the parts of speech from the language that a child hears. So, perhaps the initial experience with one language between ages 2 to 7 has a profounder effect upon a person than the second one learned after age 7. Indeed, it seems as though the true essence of a person is that of the first language(s) since it is the tongue of his/her family and native culture with its beliefs and perspectives.
On the other hand, since language is a tool that helps to stimulate the expression of human creativity, a second language, rather than the native one, may better provide a person emotional and creative expression. For instance, the Romance languages, especially Italian which has a different word for each nuance of feeling, may provide a speaker a better medium for his/her creative expression. This was one reason that the Russian aristocracy spoke to each other in French, rather than their native tongue. For, there was a refinement and a vocabulary of feeling more available to them with French. Indeed, as von Goethe observed, they could express another soul, their very personal one, in French. And, yet, they were at heart Russian always.
Tied so closely to the culture of their family and heritage, a first language remains that in which a person finds his essence and idiomatic expression. While the second language can provide creative expression, it almost always remains less that what a person truly is; for, one's first soul, or essence, is tied inextricably to one's first language.