MiniBB supports various language packs, which are available under Downloads
; each primary language pack should have a 3-letter name. Installation is easy: just copy a language file to the /lang/ folder of the forum. Some packs may also have email templates translated; email templates should have a similar 3-letter extension and copied to the /templates/ folder. As well as many add-ons may have their own language, which also should be copied either to the /lang/ folder, either as parts of the code to the primary language pack.
Keep in mind that many 3rd party language packs were created to the date of some certain miniBB release, and most likely were not supported by their authors to the current miniBB version. But certainly, until the recent release some new variables were introduced, and usually they are listed closer to the end of each language pack. So you would beed to translate them, if necessary, and preferrably send to our team to update :) The same story could be about add-ons. Many of them do not include each and every translation possible, so you will need to work it out on your own.
Note that on your forum, there should be always a default
language pack installed. In this language, the board will be displayed to Guest visitors and search engines. MiniBB comes with an English pack by default (lang/eng.php). This default language is set under setup_options.php -> $lang variable.
Even if you run multiple languages, that doesn't mean you should install a new forum copy for each language. There is always used the same software core for all languages. You don't have to create multiple pages for that.
If there is more than one language installed, any registered member could choose the language on the Profile page -> Language. As soon as it's chosen, the board will be displayed on this language.
You may test this option on miniBB.org Demo