The language pack support was added in 3.7 beta 4 BUT this version uses language packs that come in a different form. The language packs currently available on the TD website won't work with version 3.7. I have not yet updated the language packs on the TD website for version 3.7. As lolo02100 mentioned you can install a language pack from the TD website and then copy it to the correct file location.
Also, it was reported yesterday that installed language packs are deleted when you upgrade (from 3.7 beta X to another 3.7, so from 3.7 beta 3 to 3.7 beta 4, for example). So there is some work yet to be done on the language pack functionality. lolo02100, that's probably why you had to re-install the language pack.