Supported Microsoft formats are Spreadsheets (.xls) and Excel Workbooks (.xlsx). The latter is the default format in Excel 2007 and later.
The principle is one worksheet, one language per file.
On import via the API/CLI tool, the order of columns should be keyname (mandatory), translation, description and comment. When you use the web interface, you can designate arbitrary columns as again keyname (mandatory), translation, description and comment
Export is only possible in .xls format.
Plurals are not supported in Excel files.