I've learned English language on my own, and I'm sure that I'm making a lot of mistakes. Now I'm at the stage of hiring some freelancers, but I'm worried that my English will induce confusion in dialogues with developers. I'll like to find/pay someone to correct my English mistakes but I don't know how can I do that.
This is an important handicap, especially when parties on both sides of the contract aren't fluent in the English language.
Do you have any idea for my problem?
Based upon your question, I would think your English is fine.
Generalizing the question somewhat so that it is not English specific, any business dealing with multiple languages wants to do the best possible job writing product descriptions, advertisements, user's manuals etc in whatever language they use.
In the United States consumers have become accustomed to getting user manuals for imported goods that contain extremely poor English grammar, one of the prices we pay for getting cheap imported goods.
Perhaps in other countries they are less tolerant of the abuse of their languages.
If possible, it is generally a good idea to have a native speaker review your webpages, ads contracts etc. This need not be an expert, simply an average native speaker. This could be a good part-time job for a college student.
Do you know of any exchange students studying in your country who come from a country where English is the native language? Alternatively do you know of anyone from your country who is attending university in the US?
Perhaps there is a school near you that teaches English? Do they have a teacher whose native language is English?