Jar tasks beeing able to process file-sets allows the developper to specify the original manifest character encoding.
Unfortunately, although the mandatory (final) encoding is utf-8 there is no default in ant's jar task and then the original manifest processing is relying on the platform default... Windows-1252
in my case where the original manifest (coming from another jar) is truly in utf-8
Solution : specify the encoding in the task attribute
<jar destfile="final.jar" filesetmanifest="merge" manifestencoding="UTF-8">
<zipfileset src="original.jar">
[...]
</zipfileset>
</jar>