str_replace behviour is different on 2 servers

Question

I used str_replace in output buffering in order to translate a website. It works perfectly on local server and test server, but totally fails in production server.

Here is the original code :

function lang_modify($buffer){

     require('get_messages.php');

     foreach($messages as $message){
            $stringToBeReplaced = $message[0];
            $stringReplace = $message[1];
            $buffer = str_replace($stringToBeReplaced,$stringReplace, $buffer);
     }
     return $buffer;
}

$lang = $_SESSION['lang'];
if($lang=='en'){
    $buffer = ob_start('lang_modify');
}

Then, as I am translating japanese characters to english, I thought about a multibyte problem, so I replaced the str_replace with mb_ereg_replace. Again, it was working nice on local server and test server, but not on production server.

function lang_modify($buffer){

        require('get_messages.php');
        mb_regex_encoding("UTF-8");
        foreach($messages as $message){
            $pattern = "(".$message[0].")";
            $stringReplace = $message[1];
            $buffer = mb_ereg_replace($pattern, $stringReplace, $buffer);
        }
        return $buffer;
    }

In both cases, unexpected strings are being modified causing error in javascript files, and in most of cases expected replacement are not replaced. Is there any chance that the str_replace is depending to the server settings ?

I precise that I can't use another way to translate the website. I have to make this working on the production server.

Do anyone have a clue about how to fix this ?