Change default locale to system

[ChunkyProgrammer]

Change default locale to system

Pull Request Type

• Other - default setting update

Description

By default we should probably be using the language of the user's system if possible, otherwise we fallback to en-US. There were some issues with the system locale in the past which is why it wasn't made the default sooner but it seems to be working now.
I also updated the name of a variable to make it more clear that it's the fallback locale

Testing

• Backup settings.db if you need it. (make sure to backup the dev one)
• Delete the dev settings.db (make sure it's the dev one)
• Go to settings > General Settings > Language preference
• That setting should default to System default

Desktop

• OS: Linux Mint
• OS Version: 22
• FreeTube version: 0.22.0

[absidue]

We should definitely call this out as a significant change in the changelog, as English speaking users may have never changed the locale setting and might be surprised when FreeTube suddenly starts with their system locale instead.