It's very nice that you want to start reading the Bhagavad Gita, one of the best scriptures that there is. You don't need much time to read it, a chapter a day starting from the first one is also enough and would require at most 15-20 minutes of your time. Now regarding your questions:
Do I've to go through the Mahabharata ?
No, you don't need to go through the Mahabharata to read the Gita. Unless you want to know the details of the characters mentioned in the first chapter of the Gita, going through the Mahabharata is not required.
In which language should I read it?
You read it in any language you are most comfortable with. But one fact is that when the original work is translated to any other language by different persons variations do occur. So original is always the best followed by translations done by saints.
How it will change my life?
Reading Gita will not change your life unless you try to put into practice the things you learn from it. Nevertheless, Gita being the summary of the Upanishads contains unmatching knowledge and philosophy understanding which will give you happiness and peace. The second chapter of the Gita is very important as it speaks of the eternal soul dwelling in the body; and this knowledge of the soul is the starting point of all spiritualism.
If you are a spiritual seeker of you really give your mind to it, the discussion between Shri Krisha and Arjuna is so interesting that you will try to read and finish it in one go. But even after reading it, it will take time until you realize and understand it correctly.
Since you are comfortable with Hindi, I would suggest reading Gita Sadhak Sanjivani by Swami Ramsukh Das. I find that very good for any spiritual aspirant. If you just want to read the English translations you can do so at here. But just note that Bhagavad Gita - As it is was primarily meant for westerns who were not familiar or literate about our philosophy and religious tradition. Hence, it is not that knowledge rich, but good for beginners.
Also if you have an Android phone you can install this Gita App which contains translations from different saints including Sankaracharya, Sivananda, Ramsukh Das and many others. This one is very good if you want to just read the Gita verses in your desired language without reading purport, explanations etc.