All looks good now.
1 - Is Local Storage creating a file based on the key name or something like .ini or .txt file?
With NWJS export - I'm not sure, probably a file somewhere in User folder. Stackoverflow suggests it's c:/Users/username/AppData/Local/AppName/Cache
2 - Is it overwrite when I will use the same key?
Yes.
3 - Is it better than the system SAVE/LOAD?
Save/Load saves everything (excluding only objects with NoSave behavior), LocalStorage only saves one piece of data.
Like I said, the difference is like backing up your entire HDD and saving one document file.
By the way, when you save an .alon file, Language variable is saved inside of it too. So when you load this file, language in the app may change. So you might want to restore language and other settings from LocalStorage after loading .alon files.