Twitter works for a global audience, but Lolcats have consistently been ignored by the social network. Lolcats haven't been able to use the service in their native language, but a new Twitter update is correcting this injustice. If you head to the settings page, you'll see the following:
The option is effectively useless if you don't speak English, since that's all it's available in. Still, if you speak English and want to make everything on Twitter read like an image macro the settings are available. Since Caps Lock is cruise control for cool, it's also entirely in capitals.
If nothing else, it brings some attention to Twitter's Translation Center, which can be accessed here. If you're bilingual (or speak Lolcat), you can contribute.
It's worth pointing out that Lolcat doesn't affect tweets, though you likely expected this. As you can see in the screencapped tweet from @NeowinFeed (our official Twitter), it merely alters the interface around the tweet.
Adding a joke language to a service isn't all that unusual for social networks. Facebook has both 1337-style language and "Pirate" as English variants that can be chosen, so Twitter could just be following in their footsteps.