By default a Vanilla Forum is installed in English. If you want to use it on another language you need to:

  1. download the desired translation (locale)
  2. extract the downloaded file: locale.zip → DESKTOP\
  3. copy the locale folder to the locales directory on your server through FTP DESKTOP\locale_name → SERVER_ROOT/public_html/forum/locales/
  4. go to VANILLA BACKOFFICE > ADDONS > LOCALES
  5. Enable the locale
  6. and set the Default Locale the one you just enabled
  7. SAVE
When you activate a language the OneClick theme front end is also translated automaticaly to that language.

The OneClick backoffice only support some languages, but if you want you can translate it too by:

  1. going to SERVER_ROOT/public_html/YOUR_FORUM_DIR/themes/oneClick/locale/
  2. copy and existing locale file to a new one en-CA.php → NEW_LANGUAGE_ID.php
  3. you can get the NEW_LANGUAGE_ID by editing the SERVER_ROOT/public_html/YOUR_FORUM_DIR/locales/NEW_LOCALE_DIR/definitions.php
  4. and the NEW_LANGUAGE_ID is the one in 'Locale' => 'fr', in this case fr
  5. edit the SERVER_ROOT/public_html/YOUR_FORUM_DIR/themes/oneClick/locale/NEW_LANGUAGE_ID.php
  6. make the translation from english to the desired language
  7. ex: $Definition['All Discussions'] = 'Discussions Récentes';
  8. do the same for all the other translations
  9. SAVE
You can also customize the forum translations by creating a new file:
SERVER_ROOT/public_html/YOUR_FORUM_DIR/conf/locale.php

For more information please read this:
http://docs.vanillaforums.com/developers/locales/