Hi! I had the same trouble.
・http://***/LanguageCode/wp-login.php -->404 Error
・http://***/wp-login.php?lang=LanguageCode --> No translated(Almost)
Maybe , 'wp-login.php?lang=**' is use wordpress's lang file...(/wp-content/languages/**.mo),
Do you Set wordpres's lang file ?
One of the methods ,you set '/wp-content/languages/de.mo' and ' /wp-content/languages/pt.mo'.
From
http://codex.wordpress.org/WordPress_in_Your_Language
or
http://svn.automattic.com/wordpress-i18n/**/tags/**your wp Version**/messages/**.mo.
However, I looked for how to take out the login screen translated since all translation files cannot be set up.
Translated login Form (translated!)
・Login With Ajax Plugin :http://wordpress.org/extend/plugins/login-with-ajax/
I used 'Login With Ajax' + 'Widgets on Pages’
・Widgets on Pages : http://wordpress.org/extend/plugins/widgets-on-pages/
My "Login / Registration Page" is http://news.gankeijiban.com/loginr (translated!)
Wite [widgets_on_pages id="**Set Name Widgets on Pages***"]
*Sorry I'm not good Einglish.