You can improve your knowledge and ability to use the English language! Some common ways business people do this are :
Be alert. Keep your ears and eyes open, listening to how people use the language, and reading other people's work critically.
Read more. Read from business magazines, international newspapers, other people's annual reports and letters. It is also a good idea to spend some time reading fiction, as this can provide a vital freshness to your language use and increase your vocabulary. Other light reading may include biographies of famous people, especially statesmen, whose business it is to be very correct with language.
Listen to the radio on a walkman, in the car, or while relaxing at home. Radio has the advantage over TV in that it relies solely on words, whereas on TV, it is often poorly written and the visuals do most of the talking. The BBC World Service has a number of business and language programmes that may be of interest to you. Buy the BBC guide and find out.
Take some time to study an article from a business magazine or newspaper. Analyse how the sentences are constructed, the choice of the words that are used and the organisation of the data. Write out sentences or phrases that you find particularly powerful.
Buy a workbook on business writing and put one hour or so aside each week to do a number of exercises. There are many good books on the market. The Crisp series are generally of a high standard.
Take some time to learn how to use a dictionary, a thesaurus, a business reference book and a grammar book. I recommend Practical English Usage by Michael Swan (Oxford) as one of the simplest grammar references to use.
If you look up a word in a dictionary, you may like to write it out with its meaning to aid memory. Some people keep a notebook of new words : this can be very effective.