I had the problem several people experienced, and resolved it in a surprising manner: re-instalation, correctly. This could be particularly relavant to Windows, but it affected a FreeBSD system.
Problem 1: I downloaded 1.4.1, plus simplified Chinese pack, both in tar balls; unpacked them into a working version of 1.4.1RC3, on Win2K Pro. Then I uploaded the entire gallery onto a FreeBSD hosted server. The config page kept saying I hadn't downloaded any languages so only English will be used.
Resolution: After peeking into source code, I finally realized that LC_MESSAGES directory under the foreign locale (no gettext) was "Lc_messages". (It must be the WinZip used to extract 1.4.1RC3.) After correcting the case, the additional language showed up.
Problem 2: After configuring the site, I had the language selection. But nothing happens when I selected the second language. Everything was still in English. In fact, the once-working (while 1.4.1RC3) on Win2K Pro now behaved the same.
Resolution: I extracted 1.4.1 tar ball with a proper tool and the problem is gone.
Conclusion: If your hosting service allows FTP only and you are using Windows to do uploading (and expansion), make sure the tar ball is extracted properly.
Hope this helps some.