fix middleware return type

[bezhanSalleh]

Closes #79

[what-the-diff]

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.