The short answer: you can't.
The longer answer: you still can't, but I'll talk about it in more paragraphs. Ever since I have started trotting off to teach English in Asian countries, one of the most common questions from friends back home has been more of an assumption than a question. They'll ask if I speak Korean, or, now I guess, Chinese, but they seem kind of surprised to learn that the answer is no, that I am not fluent, not by a long shot, in these languages of countries where I have lived.
And by the way, I am the person who likes studying languages, who has always liked studying languages, who got the highest of my high grades in high-school German and French, who took Polish for a semester in college just 'cause, who regularly reads newspapers and books in French and Spanish and is always looking for simple German books, who has taken Korean and Mandarin classes while living abroad,. etc. etc. The spirit is willing, indeed! One of my friends said to me the first time I lived in Korea, after a few months of my being there, "If I know you, you're speaking better Korean than the Koreans by now." But I wasn't, and I still don't, and here's why: when you teach English abroad, you often find yourself working full-time in an English speaking environment, immersing your students, disciplining the children who speak in their native language. Even at my current job, teaching adults, there's an English-only policy that is indeed enforced.
Most people don't understand.how very much of my day is spent speaking English. Add to that the fact that I live with an English-speaker, and the tendency of expat teacher communities to be separate from locals in nightlife, poetry readings, arts, etc.. and you have a recipe for being nowhere near fluent in the official language of your country.
Yes, I believe you should try. You should take a class (I'm doing that) and listen to the language in various media and study a bit on your own and jabber with people in your daily encounters. I got really proficient at store/taxi/ticket counter Korean! As it happens, I would try to learn Mandarin even if I didn't feel compelled to, because I like language and languages. I just invite you to consider how good of a football player you would be if you hung around watching football practice for a year, or how well you would play the violin if you worked in the box office of symphony hall. News flash: language is a skill. It totally needs to be practiced to be acquired, just like a sport or musical instrument. You might have a talent for it, but you still have to pick up the equipment and do something to bring out that talent. Living abroad gives you chances to practice a language, of course! But not 24/7. Not when you are teaching English and being paid not to speak any other language.
I would also invite you to consider how a person from Honduras or Ecuador living in Phoenix who gets a job as a dishwasher in a Mexican restaurant and speaks in Spanish with a bunch of co-workers every day and then goes home to Spanish-speaking roommates might not magically become fluent in English overnight. Just putting that out there.
That said, my goal for June is to ramp up my language study. The first four months here were busy, chaotic, and full of out-of-town commitments. I plan to spend most of the summer right here in Guangzhou, and I am going to improve my Mandarin when I am not working. Let's check in come August and see how I did!