Sean, here comes the magic =)
You have to "tell" system what are the variables within the .php code of your plugin.
Format: OW::getLanguage()->text($prefix, $key, $vars)
Example:
Existing text key is:
File {$file_name}
{text key='mailbox+upload_file_name'}
The code within the .php file ( in this case mailbox/bol/conversation_service.php)
$language->text('mailbox', 'upload_file_name', array('file_name' => $fileList['name'][$i])) . " " . $message);
where
$prefix= mailbox
$key=upload_file_name
$vars= file_name
Structure in terms of format might vary. As you have noticed in the given example there is no OW::get part. All depends on how your plugin is developed.
If you are the type of person who can learn by comparing existing code, you can easily figure things out just by looking into existing code within existing plugins.
I am sure that the variable you would like to add to your plugin is already used in other places, just find needed text key in admin panel, copy the part after + sign in the text key. Find needed .php file and search by the copied part, you will see the needed code that you can just copy/paste.