Language can be set site wide. This really does not do a lot for your visitors, but if there is a system generated message, it will be shown in the language of your choice. (This does not change the text you enter into your website.)
For users who are logged in, each can have their own language preference. There are many more system messages shown to administrators when they are logged in, so this is more applicable to the language setting.
We are glad to add new language files for our system, but would need a volunteer who speaks the new language to check the machine translation for accuracy. We use a language editing system: POEdit. If you would like to improve or assist with our translation files, download this free program and contact us with your offer to help. We appreciate it!
You can set the time zone for your website, which will affect some calendar functions.