Kawai supports many languages out-of-the-box. By default it comes with English and Portuguese, but you can add more languages easily. To add an extra language to your wiki, you just need to modify the WEB-INF/dev/appManager.properties file:
# Languages supported languages = [ ["English", "en", "flag_usa.gif"] , ["Portuguese", "pt", "flag_brazil.gif"] ]
You can also support just one language by configuring just one, like below:
# Languages supported languages = [ ["English", "en"] ]
If you have just one language, it does not make much sense to display a little flag for it, but if you have more than one language, make sure find an image with the flag so your site looks good. Images for the flags should be uploaded as any other image through the Upload Files link.