How do I improve my English speaking skills in a very short time?

You have asked some very important questions. I think the first step is to prioritize the issues:

1. You have an interview in a few weeks and you need to perform well. This is first priority. What can you do to predict the specific skills, topics and other issues that will arise in English during the interview? How can you develop your skills to perform those unique functions? That is very do-able in 20 days. But focus: what kind of job, where, and with what kind of people? Record yourself answering hypothetical questions. See where you tend to get stuck, then answer the same question again and make it better. Repeat this exercise several times, recording each time and analyzing for patterns of triggers that make you stammer, and areas of improvement.
2. Stammering - this goes to deeper psychological/affective issues. Relax. Don't let your anxiety and self-consciousness take over. There is no need to speak quickly like in the movies. Slow, deliberate, careful speech tends to be more accurate, and gives the listener more time to process the message, especially if you speak with an accent that is different from the listener's accent. The extra processing time will work in your favor. Take a deep breath or two before you answer and collect your thoughts. Practice giving answers and pausing to think silently when lookng for a word, not stammering to fill the space with sound while you think. You can recondition yourself to develop new behaviors.
3. Finding conversation partners/friends, etc - this is more appropriate for long-term development. I don't know where you live now, other than that it is "isolated." But there are lots of online venues for "conversation/language exchange partners," or look at local universities to see if there are any exchange students who would like to help you with English and have you help them learn your language as well
4. Finally, depending on the job, you may want to actually acknowledge your "foreign language" status in the interview at some point. Many American managers would have a lot of respect for you if you said something like, "I know my English isn't perfect, and there will be a bit of a learning curve when I start out on the job, but I'm a fast learner and open to correction as needed, and I look forward to improving my communication skills and do my job as effectively as possible." (Of course, in other cultures, this could be very destructive, so you'll have to make this judgment call for yourself!)

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