You can use that style of comment across multiple lines (which exists also in HTML)
***





***

When your XML contains *&*, this will result in the text &.

When you use that in HTML, that will be rendered as &.

Here are the correct Microsoft Office MIME types for HTTP content streaming:
***
Extension MIME Type
.doc application/msword
.dot application/msword

.docx application/vnd.openxmlformats-officedocument.wordprocessingml.document
.dotx application/vnd.openxmlformats-officedocument.wordprocessingml.template
.docm application/vnd.ms-word.document.macroEnabled.12
.dotm application/vnd.ms-word.template.macroEnabled.12

.xls application/vnd.ms-excel
.xlt application/vnd.ms-excel
.xla application/vnd.ms-excel

.xlsx application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
.xltx application/vnd.openxmlformats-officedocument.spreadsheetml.template
.xlsm application/vnd.ms-excel.sheet.macroEnabled.12
.xltm application/vnd.ms-excel.template.macroEnabled.12
.xlam application/vnd.ms-excel.addin.macroEnabled.12
.xlsb application/vnd.ms-excel.sheet.binary.macroEnabled.12

.ppt application/vnd.ms-powerpoint
.pot application/vnd.ms-powerpoint
.pps application/vnd.ms-powerpoint
.ppa application/vnd.ms-powerpoint

.pptx application/vnd.openxmlformats-officedocument.presentationml.presentation
.potx application/vnd.openxmlformats-officedocument.presentationml.template
.ppsx application/vnd.openxmlformats-officedocument.presentationml.slideshow
.ppam application/vnd.ms-powerpoint.addin.macroEnabled.12
.pptm application/vnd.ms-powerpoint.presentation.macroEnabled.12
.potm application/vnd.ms-powerpoint.template.macroEnabled.12
.ppsm application/vnd.ms-powerpoint.slideshow.macroEnabled.12

.mdb application/vnd.ms-access
***

1. Save the Excel sheet as "Unicode Text (.txt)". The good news is that all the international characters are in UTF16 (note, not in UTF8). However, the new "*.txt" file is TAB delimited, not comma delimited, and therefore is not a true CSV.

2. (optional) Unless you can use a TAB delimited file for import, use your favorite text editor and replace the TAB characters with commas ",".

3. Import your *.txt file in the target application. Make sure it can accept UTF16 format.

If UTF-16 has been properly implemented with support for non-BMP code points, that you can convert a UTF-16 file to UTF-8 without losing information. I leave it to you to find your favourite method of doing so.