onboarding: Fixes bugs from #29421.

[prakhar1144]

First commit: Third commit from #30063 somehow got dropped.
Second commit: #29421 (comment)

I'm not convinced that the topics are being translated properly; they're computed outside the with override_language block. Do we want to just call send_initial_realm_messages inside a with override_language block in the caller, rather than indenting everything? Seems timely but not a blocker since none of these new strings are actually translated yet.

Screenshots and screen captures:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

Merged, thanks @prakhar1144! I think I know what happened with the third commit; I noticed I'd neglected to do a git rebase --continue after merging that PR, but didn't realize what it meant. Thanks for catching.