Improvements to Vietnamese translation

[tnhung2011]

Summary

This PR improves Vietnamese translation.

Motivation

Uploading to Weblate is possible, but since it tries to keep obsolete strings, I figure it would be better to post the translation here.

Type of Change

• Feature
• Bug fix
• Refactor (no functional changes)
• Style (formatting, naming)
• Other

What Changed

• Revised strings
• Removed obsolete strings
• Marked a string as "obsolete" (to allow for possible automated removal in other translation files)

Additional Information

Checklist

• Code changes are manually tested and working.
• Formatting and naming are consistent with the project.
• Self-review of changes is done.

• AI tools were used (if yes, I reviewed and fully understand the changes myself).

[unchihugo]

Hi @tnhung2011, editing the dictionary file directly could create merge conflicts between Weblate and upstream. Are you sure you aren't able to update the strings on Weblate instead? Also, you mentioned marking strings as obsolete, and since I can't find it in the diff, could you provide an example?

[tnhung2011]

I'm sure Weblate will automatically sync with upstream. It did before on your repo.
I can actually upload the whole file onto Weblate though. But it doesn't register removed strings.
Regarding the obsolete string, here it is:
https://github.com/tnhung2011/FluentFlyout/blob/354414ac1fa39aa22bd8ce269140b8903d54d49c/FluentFlyoutWPF/Resources/Localization/Dictionary-vi.xaml#L113