It always pays to speak and write a language correctly. If you stay in Spain or a Spanish-speaking country, or if you are constantly in the company of people who speak Spanish, you will in all likelihood learn to speak the language. But what you will be learning will most likely be colloquial Spanish, or at most conversational. When you learn a language this way, the chances are small that you will also learn the correct structural nuances and grammatical peculiarities. You will have to reinforce your familiarity with spoken Spanish with an understanding of correct grammar and construction. It is always more beautiful to hear a foreign language spoken correctly than to hear its street version. Speak Spanish the proper way. Take a learn Spanish online course.
