When using Open dialog of libreoffice calc, the non ASCII characters in a XLS document are converted correctly to UTF-8.
But when using libreoffice "convert-to" option the non ASCII characters get messed up. Googling around I found a discussion that the import filter assumes the ISO-8859-1 characters set instead of reading the contents of the file and finding the entry for Windows-1250 encoding of characters.
The command that produces this bug is:
libreoffice --headless --convert-to csv test.xls
*** This bug has been marked as a duplicate of bug 36313 ***