Java's ResourceBundle provides you a way to fallback to JVM's default locale(i.e. Locale.getDefault()) and load the properties file for the default locale, if it does not find the resource bundle for the specified locale(e.g. Czech). You can make use this API. Please go through the Java API for details.
Also, for your web application you can design a locale fallback mechanism to determine the locale for a user session (if it is required in your application).
So that the determined locale is used as a parameter to load resource bundles as well as perform any other locale sensitive operations.