Here are two small utilities to convert between TikiWiki's language.php files and GNU gettext PO files. PO files are generally easier to translate since a number of tools are available to automate this.
Moreover, the Translate project has produced Pootle — a web interface that allows anyone from the net to translate PO files. No need any more to be initiated in the black magic of PO file manipulation.
The utilities should make it easier to pick up on a stale Tikiwiki translation.
By installing the Translate Toolkit you'll get this script automatically, however, the souce is available from SVN should you want it separately: tiki2po and po2tiki.