I checked the Turkish translation and apparently the key for this message is translated, as follows:
<msg_key>SFSM_UserDeactivated</msg_key>
<msg_language>1055</msg_language>
<msg_text>{\rtf1\ansi\ansicpg1252\deff0\deflang1033{\fonttbl{\f0\fnil\fcharset162 Tahoma;}{\f1\fnil\fcharset0 Tahoma;}}
\viewkind4\uc1\pard\lang1055\f0\fs18 Bu hesap \'feu anda aktif de\'f0ildir. Sistem y\'f6neticisine ba\'fevurunuz.\lang1033\f1\par
}
Are you sure that by the time the login form is presented to your user, the MessageLocaleID is set to 1055?