Presuming that your text file is called replace.txt and that it's in your mIRC directory, give this a try.
Code:
 alias replace.word {
  var %a = 1
  while %a <= $0 {
    var %word1 = $gettok($1-,%a,32)
    var %word2 = $iif($read(replace.txt,s,%word1),$v1,%word1))
    var %line = $replacex($1-,%word1,%word2)
    inc %a
  }
  $iif($isid,return,echo -a) %line
}

Usage: /replace.word <sentence>
or $replace.word(<sentence>)

The first usage will return the information as an echoed line in your active window.
The second usage will return the information to the calling script.

Change filename if necessary