There a single file which need to translate, the definition is making using “constant”.
Files are located in the “language” folder, here the english.

In this case it's not like in btitracker v.1 using constant but using a single array and it's multiple file:

• install_lang/install.english.php
• lang_account.php
• lang_admin.php
• lang_blocks.php
• lang_forum.php
• lang_history.php
• lang_login.php
• lang_main.php
• lang_news.php
• lang_peers.php
• lang_polls.php
• lang_recover.php
• lang_smf_import.php
• lang_torrents.php
• lang_upload.php
• lang_usercp.php
• lang_userdetails.php
• lang_users.php
• lang_viewnews.php