Translations, also known as "language packs" or "languages" are available to translate the MyBB interface into another language. All of the text that MyBB generates (besides user-inputted messages), are translatable. The ability to do this lies in the various .lang.php files found in the inc/languages/<language name>/ folders.
Each MyBB file has its own .lang.php file. For example, the language phrases used in member.php would be stored in inc/languages/<language name>/member.lang.php.
There are also some "global" language files that are used in all of MyBB. These include global.lang.php and messages.lang.php.
Each board comes with the English language pack by default. It is recommended that you do NOT delete this language pack.
The complete translations that have been submitted to the MyBB Group are listed on the MyBB translations page.
More discussions about translations (in development, or other), may be found in the Internationalization forum.
There are usually two main components of installing a language. Firstly, the language files need to be uploaded. Secondly, the translated image files (should they exist) need to be uploaded.
Uploading the language files
The language files should be uploaded to the inc/languages/<language name>/ folder. The translation author should give you instructions via his or her readme on details including what the <language name> folder name should be.
One file remains in the main inc/language folder, and that is the "meta" language file that contains information about the language.
Uploading the image files
If these exist, they should be uploaded to images/<language name>/ and groupimages/<language name>/. Again, the translation author should have given you more detailed instructions in his or her readme file.
Open up your User CP Edit Options page and you should see the translation in the "Other Option" menu. Try out the new translation and make sure that everything works.