Hi, Bernhard Reiter, see the patch file as attachment, this patch is
against the roundup 1.5.1 release source.

I just extract Chinese substring from the whole text, and split it to
several Chinese words by mmseg. Then I add each term for each Chinese
words. The whole text is still parsed by the default English indexer, so
that we can index all the Chinese and English words.

Please note that in the html file issue.item.html, the wrap option
should be (in the line 89)

wrap="soft"

Otherwise, the Chinese sentence will be divided mistakenly by the added
line breaks. See my report here: [Roundup-users] hard line breaks were
automatically added between Chinese chars when I add a message -
https://sourceforge.net/p/roundup/mailman/message/34879502/