Use macOS APIs to get preferred localization and keep defaults as fallback

[sdassow]

Newer versions of macOS allow to set a language per application instead of only system wide.
This currently affects Fyne, and is part of this issue: fyne-io/fyne#5277

The suggested solution is to use the macOS API instead of just the defaults command.

[sdassow]

Going to add the plural version as well...

[andydotxyz]

@jeandeaual do you need anything else added to this PR to allow the workflow to run or a review?

My apologies for the prod on Simon's behalf.

[jeandeaual]

Apologies for the delayed response, and thank for your patience.

Overall, this looks great to me.
I did notice that the API returns different locales compared to the system-wide ones, and it doesn't include a region (at least in my case), but this shouldn't be an issue.

@sdassow The CI/CD pipeline was is a bit out of date and failed during setup. Could you please rebase your branch on master and push it again?

[jeandeaual]

Thanks for the contribution!