Reader Spaces: track language settings added to spaces

[gabrielcaires]

Context

Follow-up for #112046. That PR adds a Languages field to Reader Spaces create/customize flows, threads languages through the read-spaces API types/adapters/mutators, and refreshes both feed/discover streams when a space changes.

We should cover these new language settings in Reader Spaces analytics separately from the initial implementation.

Scope

• Decide which Reader Spaces events should include language metadata after PR 112046 lands. Likely candidates:
  • calypso_reader_spaces_page_viewed
  • calypso_reader_spaces_space_created
  • calypso_reader_spaces_space_updated
• Add stable event properties for the selected languages, using the base language codes introduced by the PR.
• Consider whether to track exact language codes, a count only, or both, based on analytics/privacy expectations.
• Ensure spaces without saved languages and spaces with unknown preserved language codes degrade predictably.
• Add focused tests for create, update, and page-view event payloads.

Acceptance criteria

• Analytics payloads account for the Languages field introduced in PR 112046.
• Tests cover saved languages, empty/missing languages, and unknown preserved codes where applicable.
• Event names/properties are documented or easy to discover from the tests.

Related

• PR: Reader Spaces: let users choose a space's languages #112046
• Original PR issue: RSM-4510

[bolitaria]

I've opened a new PR that implements the languages array tracking.
Ready for review: #112932