I am french an my Windows system and web browser are set in french language. So when I am typing here (here = typing a question or an answer in BSE) nearly all the words are underlined and that does not help... How to set it in english ?
As gandlaf said in the comments, SE does not have any spell checker built in to the question or answer fields. All spell checking is handled by your browser. (Or if you are using a browser such as, say IE before version 10, you will get no spell checking.)