In order to be Unicode compliant, we are using one "special" mode of the PEAR excel library that allows to write UTF-16LE, so we can convert utf8=>utf16 and write them properly.
The problem is that such "special" mode is some unestable for some big sheets (XLS limits AFAIK). I've done tons of tests and sheets are broken randomly, depending of the contents.
The proposal is (there are two):
1) Allow one admin setting to go back to the "default" mode (iso-8859-1). It will help to all the iso-8859-1 compliant langs (PEAR excel library rocks under iso). Could fit easily inside 1.7.1 and above.
2) Build one "OpenDocument" alternative. I've been doing one small research across Internet and it seems that there isn't any product ready yet. Perhaps we could build it (shouldn't be really difficult in theory, It's XML) and help the whole PHPers Community! (1.8...)
If approved, I would split this into two subtasks.