Watchdog and translations
As per the core documentation we should use the watchdog function without the t() because it is translating when reading and outputing the log message.

Watchdog invokes dblog_watchdog() in "modules/dblog/dblog.module" and dblog_watchdog() is not translating anything before writing into the database BUT it is translating when reading and outputing the log message.

Reference:


https://api.drupal.org/api/drupal/modules!dblog!dblog.module/function/dblog_watchdog/7  
https://api.drupal.org/api/drupal/modules!dblog!dblog.admin.inc/function/theme_dblog_message/7  
https://api.drupal.org/api/drupal/includes!bootstrap.inc/function/watchdog/7