GeSHi is extensively used on numerous MediaWiki sites, so it is rather paradoxical that it does not support wikitext.
Xavier de Pedro
... As well as in Tiki sites:
That support would be awesome!
If you know a little php, it would not be hard to make it yourself. See section 4 of the documentation that comes with GeSHi.
Otherwise, I'm not familiar with this language and so could not write the language file myself. If you could find someone with a little PHP experience and who knew your language, you could get them to write it.