Since version 1.4, Gallery has supported many different languages. In 1.4, we shipped Gallery with ALL languages in the release. However, in Gallery 1.4.1 and later, we distribute each language individually in a language "pack." Gallery's native language is English; however, if you'd like to use Gallery in another language, you must download a language pack and install it. This is advantageous because you can pick which language or languages you want Gallery to be displayed in. This also reduces the size of the general release package.
You can download language packs for the most recent release on The Gallery SF.net Download Page. Pick the language you want, download it, and untar it. You will get a directory that looks something like [language]_[COUNTRY]. Just upload this directory (in BINARY FTP mode) to the locale/ directory of your Gallery installation. After that, simply rerun the configuration wizard to enable the language.