phpBB 3 Language Pack
How to install a language pack in phpBB 3
Installing a language pack in phpBB3 can be done with just a few clicks. Go to your forum's admin area, select the System tab from the upper menu and click on the Language packs link on the left.
Once the language is installed, select the General tab from the upper menu and then click on the Board configuration link from the left menu. Then from the drop-down menu next to the Default Language setting select the newly-installed language and click on the Submit button at the bottom of your screen.
Now when you visit your forum it will be displayed in the newly-installed language.