Allow args in i18n

i18n function should be able to handle args, for example when a file name is required. Something like: `There's already a file named %{name}%`. In some languages it could be written differently so I added this to the core. Now language files could contain something like: 
```php
$lang["There's already a file named %{name}%"] = "Esiste gi&agrave; un file chiamato %{name}%";
```