new language[MAX_PLAYERS];
#define DIALOG_LANGUAGE 911

// Depends on where and when you want to show the dialog.
ShowPlayerDialog(playerid,DIALOG_LANGUAGE,DIALOG_STYLE_LIST,"English\nOtherLanguage\OtherLanguage","Select","Cancel");

// OnDialogResponse
switch(dialog_id)
{
    case DIALOG_LANGUAGE:
    {
        switch(listitem)
        {
            case 0:
            {
                language[playerid] = 1; // Let's say ID 1 language is English.
            }
            case 1:
            {
                language[playerid] = 2; // What language you want.
            }
            // and so on
        }
    }
}
// OnPlayerDoesSomething
switch(language)
{
    case 1:
    {
        SendClientMessage(playerid,-1,"Selected language English.");
    }
    case 2:
    {
        SendClientMessage(playerid,-1,"adsaidsaidsaidad");
    }
}
// Don't forget to reset the variable when the player disconnects.
public OnPlayerDisconnect(playerid, reason)
{
      language[playerid] = 0;
}