Great question, Steve!!
I had a strong hunch, but I wanted to know for sure so I tried to find a list of three-letter language codes. I couldn't - so I ended up just following my hunch to see if I was right and to see the documentation reference in the explanation.
I turned out to be right, but the reference leaves me wanting more. It points to a list of language codes in the format xx-YY (four letters seperated by a dash), not the three-letter format required for thesaurus files. It appears as if the three-letter format is always found by removing the dash and the last letter from the listed language code, but this is not described on that web page. And after following the link on that page to http://msdn.microsoft.com/en-us/goglobal/bb896001.aspx (which is listed as being documentation for Windows Vista!), I see a table that suggests that this is not the case - but that also includes many languages that I believe not to be supported by SQL Server, so I'm not sure how relevant this is.
Can anyone fill me in on the missing details?