the language keys are not in the __key__ format. They are _key format.
the items you see in an .html file like __key__ are actually variables from an sql query that is within the code. Depending on what you are trying to display will determine where in the code it is at, and/or where you need to add your variables.
Example. WHen looking at your account page, the Account block has a bunch of information in it. If you look at the thumbnail_single.html file, you will see a variable as __user_title__. This variable comes from the BxBaseFunctions.php page. It is called from a database query. Similar to what I have below. Mine is different because I have changed my account block and added new variables, but I hope it helps you unsderstand what is going on
$aVariables = array(
'iProfId' => $iId ? $iId : 0,
'sys_thb_float' => $sFloat,
'classes_add' => ($bGenProfLink ? ' thumbnail_block_with_info2' : '') . ($sType != 'medium' ? ' thumbnail_block_icon2' : ''),
'sys_status_icon' => $sStatusIcon,
'sys_status_title' => $oUserStatusView->getStatus($iId),
'usr_profile_url' => $sLink,
'usr_thumb_url0' => $sThumbUrl,
'usr_thumb_title0' => $sUserTitle,
'bx_if:profileLink' => array(
'condition' => $bGenProfLink,
'content' => array(
'user_title' => $sUserTitle,
'user_address' => $aProfile['Address'],
'user_address2' => $aProfile['Address2'],
'user_city' => $aProfile['City'],
'user_state' => $aProfile['State'],
'user_zip' => $aProfile['zip'],
'user_phone' => $aProfile['Phone'],
'user_email' => $aProfile['Email'],
'member_id' => $aProfile['ID'],
'user_info' => $sUserInfo,
'usr_profile_url' => $sLink,
),
looking at the above code, you will see 'user_title' => $sUserTitle,
the 'user_title' variable is in the html file is the __user_title__