I have to say fluency in business English is likely to be very important for BI Analyst. As a BI analyst, your business users are VP and managers (at least mine are). You have to be able to communicate with them effectively. I find reading books on business letters, especially business emails can be very helpful.
I borrowed the book titled "300 successful business letters for all occasions" by Alan Bond and Bancy Schuman. I find it is very useful for my daily email communications. I also borrowed a few other similar books from local library before - they are all quite good.
Depending on the working environment, sometimes companies are more tolerant with imperfect written English than others. But it is always good idea to grasp the style of business communication before entering the job market.
Jing