I have attached a very simple tool to help determine which strings are missing from language files. To use it, place Language.php in a new web/services/Test folder under your VuFind installation, then access with a URL like this:
The lang1/lang2 parameters are the names of two languages represented in the web/lang folder.
The output will show a list of strings that exist in only one of the two files.
I do not claim that this is pretty code -- it's a quick hack to do the job -- but hopefully it will save somebody a little time.
Edit (7/14/10): I have commented the code, tidied the HTML slightly, and added a feature to show translation percentages (vs. English).