You can edit the content of the directions file manually, since those are in plain XML. The file you need to localize is located under %ArcGIS Installation folder%\Desktop10.0\NetworkAnalyst\Directions\resources.xml.
The Supported Languages (codes) are:
English (en-US) - default language
As far as I understand, this is only for managing the non-Latin symbols properly because you don't have to change the "Language ID" tag's value when making edits to the directions tags' text values.
However, this is a pretty big and tedious task - to translate all the text in the tags and make sure it makes sense when routing. So, as Alan from Esri mentioned earlier, it is best to contact a local Esri distributor and if you are on maintenance you should have no problem obtaining a translated directions file which you then could use.