Allow creating posts with no language

[micahmo]

Pull Request Description

This PR updates the Create Post page to allow setting no language when creating new posts. This matches the web UI and is allowed by the Create Post API. Note that setting no language (null) is different than setting Undetermined (language id 0), although they both display no language name next to the post in the web UI. This also restores the previous behavior of Thunder, which always used to create posts with no language in the past.

Issue Being Fixed

Issue Number: Related to #922 (not the full fix yet)

Screenshots / Recordings

Vw7uMiVouU.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

Note that setting no language (null) is different than setting Undetermined (language id 0), although they both display no language name next to the post in the web UI

That's an interesting observation! Nice catch there - I didn't realize null and Undetermined were two different options.