As you have noticed English its not my mother tongue so the private messages plug-in has some errors.
Here all the strings that appear on it, for correct them and use it to translate the plug-in to other languages.
#: messages_init.php:26:
"You are authorized only to view your own messages"
#: messages_view.php:26:
"Delete"
#: messages_view.php:27:
"Mark as unread"
#: messages_view.php:28:
"Mark as read"
#: messages_view.php:30:
"Inbox"
#: messages_view.php:31:
"Send by"
#: messages_view.php:38:
"Sent to"
#: messages_view.php:39:
"Sent messages"
#: messages_view.php:59:
"Back"
#: messages_view.php:71:
"Next"
#: compose.php:16:
"Messages"
#: lib.php:15:
"Your Messages"
#: lib.php:29:
"View Messages"
#: lib.php:34:
"Compose"
#: lib.php:39:
"Sent Messages"
#: messages_message_detailedview.php:40:
"Reply"
#: messages_message_detailedview.php:41:
"Are you sure you want to permanently delete this message?"
#: default_template.php:11:
"Sent by"
#: default_template.php:12:
"Date"
#: default_template.php:15:
"Action:"
#: default_template.php:16:
"Select an action"
#: default_template.php:17:
"Are you sure you want to permanently delete this message(s)?"
#: default_template.php:24:
"Message List"
#: messages_new.php:32:
"(Community)"
#: messages_new.php:38:
"No friends avaible for send messages!"
#: messages_new.php:63:
"Sent a message"
#: messages_new.php:64:
"From:"
#: messages_new.php:65:
"To:"
#: messages_new.php:66:
"Subject:"
#: messages_new.php:67:
"Message:"
#: messages_actions.php:65:
"The selected message was deleted."
#: messages_actions.php:67:
"The provided message id its not valid!."
#: messages_actions.php:148:
"You have received a message from %s."
#: messages_actions.php:150:
"To reply, click here: %s"
#: messages_actions.php:155:
"You cannot reply to this message by email."
#: messages_actions.php:156:
"Failed to send email. An unknown error occurred."
#: messages_actions.php:163:
"Your message was sent"
#: messages_actions.php:168:
"You coudln't send a empty message!"
#: messages_actions.php:171:
"You must to specify the sender and recipient for send a message!"
Keywords: i18n, internationalization, plugin, private messages

Comments
Thanks a lot!
I already update those files and upload the new version.
I added those strings in a file called strings.txt that its in the po format so if you want to make translations to your current language only need to copy and paste those strings in your language .po file and make your own translations.
I installed the messages plugin based on the directions and I can see the "your message." When I attempt to compose a message the "To:" field is blank. It does not provide me the option to enter a user or select a user from a list.
How can I fix this?
I believe that this is explained here:
http://elgg.org/mod/forum/forum_view_thread.php?post=23
Currently you can only send to your friends or communities. If you have no friends list or are not a member of a community, there is no one to send to.
But I think that this may be changed in a future version, correct?
Just curious, I think I installed 0.1.2 from this website. I had two "success" messages up top when I reloaded the page - the "Your messages" appears in my bar. Now I got two issues:
1) I translated strings.txt into my language - do I need to place this somewhere other than mod/messages ?
2) Messages are not sent - don't appear in another INBOX and not in SENT. What can I do to double check everything is setup correctly?
Hi till,
For the translation thing you must to copy and paste the strings that you translate into your language file and install it. For more information you can see: http://elgg.org/mod/mediawiki/wiki/index.php/Internatio
Please check the prefix_messages table if your messages are there.