The small xx represents the local language, which is a two letter ISO-639 language code and the capital XX represents the local country, which is a two letter ISO-3166 country code.

小写的xx 表示本地语言，它是一个 ISO-639 语言编代码，为两个字母；大写的 XX 表示本地国家，它是一个 ISO-3166 国家代码，为两个字母。

youdao

I have a small mouth and beautiful cherry face, my hobby is XX, I often see the work of customs declaration, think they are too great, so ready to sign up customs broker.

我长着一张樱桃小嘴和漂亮的脸，我爱好是XX，我经常看报关员的工作，觉得他们太伟大了，所以准备报名报关员。

youdao

Small world, I am from XX too.

世界真小，我也是XX人。

youdao

Alas, his uncle has no idea of the right size: small, medium or even XX-large?

ECONOMIST: Johnson