How to install language packs in phpBB3
Every phpBB3 installation comes prepackaged with the English language pack. If you wish, you can install many more languages.
First you have to download the language pack that you want from the official phpBB site. At present there are more than forty languages to choose from.
After you download the archive to your local computer you have to unzip it. It contains a couple of folders: language and styles. The language folder contains a folder with the ISO code name of the language (e.g. fr for French, es_x_tu for colloquial Spanish, etc.). You have to upload this folder to the language folder in the root phpBB3 directory on your hosting account (e.g. public_html/phpbb3/language). You can upload the folder with an FTP client.
The styles folder in the downloaded language pack on your computer has some images which are for the two styles that come prepackaged with phpBB3: prosilver and subsilver2. Upload only the folder with the ISO code name to the imageset folder of the respective style on your account (e.g. public_html/phpbb3/styles/imageset). These images are not something crucial that you need for the installation of the language pack.
After you upload the folder from the language directory on your local computer to the language directory on your account, you can proceed with the installation. Log in to the administration control panel of your phpBB3 board, click on the System tab and then on the Language packs link on the left. The language pack you just uploaded should be listed under Uninstalled language packs. Click on the Install button to complete the installation. Once you do it the language will be listed in the same table, but this time under Installed language packs.
You can change the default language for the board from the administration control panel: General tab>Board settings. This will change the default language for all guests who visit your board. Registered users can choose their default language from the ones you have installed.
For some more details and screenshots you can also visit the phpBB3 languages tutorial.