It would be interesting to have Gnome Subtitles automatically detect text language, both for subtitle text and translation. That way the respective options in the Tools menu could be automatically set.

LibEnchant (used for spell-check) doesn't seem to support this. There are also online services available (example: http://stackoverflow.com/questions/1192768/return-the-language-of-a-given-string/1192802#1192802), but a local approach seems better. The following library might do the trick: http://ntextcat.codeplex.com/ (NTextCat)

Other references:
http://code.google.com/p/language-detection/ (Java)
http://textcat.sourceforge.net/ (Java)