Write for the Reader
When you know a subject well, it is surprisingly easy to forget not everyone knows what you do. The history, the background and the terminology may all be familiar to you, but an English-speaking client or partner may need better and clearer information. That takes a proper knowledge of spoken and written English.
A simple translation is usually not enough for professional documents. Nuances in dialect, speech patterns, context and spelling can make a vast difference between an English translation and a document written professionally in English.
Make it Clear
Acronyms, abbreviations, codes, numbering, lettering, rules and norms may all be very useful to someone in your organisation but can leave a native English speaker wondering if this is actually going to work in different languages.
There is often no need to explain everything in detail. If someone is dealing with your company, they will probably already know a little about what you do and understand the basics of your organisation. Your responsibility is to ensure they understand everything you have written.
Decide what matters
Keep your writing professional but friendly. You are writing for them, not for yourself, so make sure you include what you need to and omit anything that doesn't have a direct influence on your conversation.
Make everything as clear as you can, especially important issues. Deadlines, dates, times, communication methods and addresses may sound like common sense, but a postcode, for example, is written in many different ways.
Intention Doesn't Count
What you intended to say and what the reader actually understood are not always the same. If in doubt, put yourself in their shoes and see if you know what you are talking about.