Apparently, segmenting your customized software into easily-updatable Play Store apps is a popular trend. HTC is the latest to get on board, presumably because the person who makes the keyboard work is tired of waiting on the whole Sense team to put an over-the-air firmware update together. HTC published extra language packs back in April, and now Sense users can get timely updates for the keyboard as well.
You know the drill: this will only work on compatible HTC hardware, so don't even try it on other phones or tablets. I don't have much in the way of HTC history connected to the Play Store, but I'm guessing that the keyboard app is only for phones that are on the current update bandwagon, which means the One family (2013 and forward), the latest entries in the Desire line, and perhaps a few high-end handsets from just before. Users may see the app appear on their phones automatically.
HTC is showing off an Asian input option (Chinese, I'm guessing, but I could be wrong) in the Play Store, but don't worry, Arabic, Cyrillic, and various other alphabetical characters are in there as well, at least if you've got the aforesaid language packs. Or you could just use Google's keyboard and not worry about it.