fix middleware return type

PR Summary

Locale Handling Method Update The method which handles language localization in our system, found in SwitchLanguageLocale.php file, has been updated. This change allows it to accommodate a wider variety of return types, increasing its flexibility in handling different situations during its execution.
Locale Value Assignment Update We have refined the way the variable $locale(which represents chosen language for user interface) is set. It now uses the null coalescing operator, a safer programming practice. This ensures that a default value is automatically used if the usual sources for the desired language setting (session or query parameters) do not provide a value.