[OSPR-5979] Port "LANGUAGE_CODE" site configuration option to Lilac

[0x29a]

Description

This PR contains changes from https://github.com/edx/edx-platform/pull/27696 PR cherry-picked to open-release/lilac.master branch.

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[pomegranited]

FYI @arbrandes :)

[cmltaWt0]

@pomegranited Going to test it.

Diff is simple and clear - I want to test some corner cases to ensure it works as expected.

[cmltaWt0]

Notes so far

• legacy UI is translated from the first point of view
• MFE Account application doesn't get new language prefs

So presumably some effort should be dedicated to MFEs to work with this change.

P.S. it's a quick review and I'll will do some deeper tests:

1. Check possible BackboneJS + Underscore templates parts to be compatible with the changes.
2. Review the MFE logic a bit closer to ensure it's not a configuration issue.

[cmltaWt0]

Additional finding.

Proposed port is not complete and doesn't change some JS (Backbone) default language logic:

In case we change site language account settings we will get the correct behaviour:

So as a conclusion- this PR should be enhanced a bit to be a complete solution.
However we can merge it and get almost complete default language behaviour but we must add Known issue as a part a this PR as I describer above.

@arbrandes @BbrSofiane what do you think?

[natabene]

@0x29a Thank you for your contribution. I see it is already assigned to @arbrandes .

[arbrandes]

@cmltaWt0, thanks a lot for the detailed review, including testing on Lilac. Here's what I think, though:

1. We need to post this to the forum and wait to see if there are any objections to us merging this to Lilac. It is a new feature, after all, and I fear it falls under the "useful but too few users" category, which means somebody might object to it.

2. It looks like it might indeed be incomplete - or that the platform itself has some unrelated translation bugs. I've asked the original authors to investigate.

[arbrandes]

Opened discussion about this in the forum, as per the BTR group's ADR on merging backports.

[BbrSofiane]

@edx-community-bot merge

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