Watchdog messages and string replacement
Just use this:

watchdog('mymodule', 'Field !field_name created.', array('!field_name' =&gt; $file_field_name));


watchdog() function will translate the string you pass as the second parameter. You shouldn't translate the message before passing it to the watchdog().

Check out this answer for more information and references.

Or consult the following document from Drupal's documentation.