Book excerpt: Write like you speak and speak like you write
Obviously writing a business report, writing an essay on philosophy and writing an academic paper all require you to organize your information in different ways. You need to be more formal and structured than when you engage in casual conversation. The preferred structure for such writing will even vary from culture to culture. However, whenever I wrote in a foreign language, and French was my first, I felt that the individual sentences I wrote were the same as my spoken language. In my mind I made no distinction between the written and the spoken language, even though there undoubtedly was one. I always tried to make them both as similar as possible. I recommend this approach to all language learners as a way to improve the accuracy of both your written and spoken language.
I believe that is not helpful to divide a language into categories to be learned as separate skills. Today there are courses offered in business language, academic language, technical language and so forth. To me language is one. To be effective and persuasive in a new language you have to build a solid base of key phrases and words that you can use comfortably in speaking and writing. If you have that base, technical vocabulary is easily acquired. If you can express yourself clearly and logically, you can easily learn to write business reports or academic papers.