I visit library.gnome.org by Firefox and Chromium. It's English at first, and then, I clicked "中文 (中国大陆) (zh_CN)" from "Available Languages:" section, the page is correctly shown in Chinese.

However, whatever link I clicked on the page, say "http://library.gnome.org/devel/references", the page will become English again.

And I found this message right below the "Available Languages:" section: 

"Unable to display document in preferred language loaded from cookie, as translation probably does not exist.

Remove cookie"

I tried remove cookie and click zh_CN again, it just show the current page in Chinese, for any other link, it will turn back to English.

I also checked the link, http://www.cs.tut.fi/cgi-bin/run/~jkorpela/lang.cgi , which is used for detect the Accept-Language section. For the firefox, it returns:

Accept-Language: zh-cn,zh;q=0.5
	zh-cn 	1
	zh 	0.5

For Chromium:

Accept-Language: zh-CN,en-US;q=0.8,en;q=0.6
	zh-CN	1
	en-US	0.8
	en	0.6

The language setting is definitely correct, so it should be a bug in the language detection part of library.gnome.org.

Could you fix it? Thanks.