enum PlayerStats{
...
PlayerLanguage,
...
};

if(IsPlayerLanguage(playerid, english)) ShowPlayerDialog(playerid,1,DIALOG_STYLE_INPUT,"Login","Enter your password below:","Login","Cancel");

if(IsPlayerLanguage(playerid, dutch)) ShowPlayerDialog(playerid,1,DIALOG_STYLE_INPUT,"Login","Vul hier je wachtwoord in:","Login","Beindigl")

IsPlayerLanguage(playerid, lang)
{
    if(lang == 'english') {
        if(PlayerInfo[playerid][PlayerLanguage] == 0) return 1;
    }
    if(lang == 'dutch') {
        if(PlayerInfo[playerid][PlayerLanguage] == 1) return 1;
    }
    return 0;
}