For example, you can use this CSS when you want to hide XX only on the English version of the site.

:lang()
The :lang pseudo-class can be used when you want to apply a language-specific style, for example, when lang=”en-US”.

div#notice-area-wrap:lang(en-US) {
display: none;
}
The notice-area-wrap is now hidden!

It’s visible on the Japanese version of the site!

Summary
You can apply a style by specifying :lang() language. Very useful 😃.