There's a saying which roughly goes, "The number of languages you know is the number of times you are a person." ("Koľko jazykov vieš, toľko krát si človekom" in Slovak, but other languages have a similar expression.) Basically, as you learn an extra language, you also develop a personality in accordance with that language when you speak that language. It won't be a complete Jekyll-and-Hyde difference, but for example, if one language expresses itself quite directly, then as you speak that language your behavior may become more direct as well. An indirect language (when English is spoken properly, it is surprisingly indirect) would lead you to approach a situation indirectly. I suppose there's some cultural influence here, but that's the basic idea. I can't agree much with Julio's explanation, simply because in his answer the question is approached from the perspective of technical proficiency. Taking such a perspective would be quite misleading.

According to what I have understood, language is the identity that you have with a second language that you are learning. If you are familiar and confident with it, or, for the contrary, if you can't express yourself properly. An example of a person with a low level of language ego is someone who does not feel comfortable with the pronunciation of a sort of words and have some inhibition. On the opposite situation, someone with a high level of language ego uses a long-range vocabulary, speaks with dexterity and does not have fear of committing mistakes. http://polyglossic.wordpress.com/2012/07/31/applied-linguistics-language-ego/