It seems that plurals in language/en/ucp.php

'FOLDER_MESSAGE_STATUS' => array(
1 => '%2$d out of %1$s stored',
2 => '%2$d out of %1$s stored',

are build differently (the part in parentheses) than here

'FOLDER_STATUS_MSG' => array(
1 => 'Folder is %3$d%% full (%2$d out of %1$s stored)',
2 => 'Folder is %3$d%% full (%2$d out of %1$s stored)',

In my language the first one is correct and second not.

Second issue is with the part "%1$s stored" - it is showed for example as" 5 messages stored". In my language we use declension and because that part ("5 messages" ("%1$s")) is loaded from file language/en/common.php

'MESSAGES_COUNT' => array(
1 => '%d message',
2 => '%d messages',
),

it is incorrect.

So I would like to ask if you can add into en/ucp.php separate

'MESSAGES_COUNT_XY' => array( ), just for counting of PM's.

Hope it make a sense