I found with Korean fonts, "bg" seems to be chosen as the language for sample text. I think it's because "bg" is alphabetically first in the supported language list.

$ fc-query /usr/share/fonts/truetype/baekmuk/batang.ttf |grep lang
	familylang: "en"(s)
	stylelang: "en"(s)
	fullnamelang: "en"(s)
	lang: bg|fj|ho|ia|ie|io|ko|kum|nr|om|os|ru|sel|so|ss|st|sw|ts|uz|xh|zu|kj|kwm|lg|ms|ng|rn|rw|sn|za(s)
$ 

Fonts don't seem to have information to finding out their representitive language. So how about just putting language for sample text in metainfo file?