actually.. I noticed it does work.
The problem is indeed that my page use Julia multiline comment syntax ("#= [..multiline comments..] #=") and that make Geshi to crash and not render the highlight.
I did try to change the julia.php file with 'COMMENT_MULTI' => array('#=' => '=#'), but still doesn't work.
I think is some bug in Geshi, and as it seems a dead project, I think it will not be solved.
Maybe it's time for Dokuwiki to update its syntax highlight engine (e.g.
http://prismjs.com) ??