Only display Preview Language Setting (dark_lang) in LMS

[kdmccormick]

Implemented by #36271

See that PR for rationale and details.

Update, September 2025: The PR linked above is stalled. There's a unit test failing which I haven't been able to figure out. I'm open to anyone who wants to help diagnose and fix it.

[salman2013]

I am updating this ticket from small to medium, as it took additional time to investigate the failing tests.
Initially, I attempted to fix the tests using different approaches, but realized that they are no longer applicable due to the recent changes. Since the endpoint now redirects, the expected session behavior is not established, making the existing tests ineffective.

[salman2013]

I have closed the stalled PR #36271
Created new PR: #38510

[salman2013]

I have closed the stalled PR #36271
Created new PR: #38510

[kdmccormick]

Closed by: #38510

It looks like dark_lang doesn't apply to MFEs, but that's an existing issue which is out of scope of this ticket. Captured as future work: #38626