I've tried to do an upgrade on a 1.7.3 site and never seen this error before:
The language files needed for your current language selection () are unavailable. Please put the language pack in this folder: expressionengine/language/
I've tried upgrading to 2.6.1 and 2.5.5 and both give the same error.
Language files are definitely in place.
Can't find any refs online.