Discovered the following updating one of my addons last night:
SendChatMessage() no longer accepts capitalized language names as a way of specifying what language the character is using. By intercepting the function and using the chat menus to change languages, I discovered that it is now being passed a number specifying the language to use.
The language numbers can be obtained from GetLanguageByIndex(), which now has two return values, like so:
index 1 appears to still be the faction language, while index 2 is the character's native language, for those whose faction language is not their native language. GetNumLanguages() still returns the number of languages the character knows, which is the maximum index for GetLanguageByIndex; thus, you can walk the list of languages the character knows, getting their names and numbers.
If you attempt to pass SendChatMessage() a string with a language name in the old style, the character uses their faction language.