Currently, the REL1_43 branch of Wikibase includes almost all the functionality of the mul language code, but gated behind a feature flag called tmpEnableMulLanguageCode. On the master branch since the 1.43 release, not much further MUL work happened, but the feature flag was renamed to enableMulLanguageCode (T330217). In a call between @Anton.Kokh, @roti_WMDE, @jon_amar-WMDE, @Lydia_Pintscher, @Tarrow, and @Lucas_Werkmeister_WMDE (internal notes), there seemed to be agreement that it would be worth backporting this rename to REL1_43, so that we have a complete, stable, supported MUL feature in this LTS release.

We almost certainly want to backport:

• Remove tmpAlwaysShowMulLanguageCode temporary setting
• Rename the tmpEnableMulLanguageCode flag to enableMulLanguageCode

We might also want to backport:

• Move "mul" to the top of the termbox (T371802) – this appears to be the only other MUL work that happened after the REL1_43 branch was cut; we might as well include it for parity with Wikidata.

Acceptance criteria:

• In a Wikibase installed from the latest REL1_43 branch (or 1.43 release), setting $wgWBRepoSettings['enableMulLanguageCode'] = true; + $wgWBClientSettings['enableMulLanguageCode'] = true; enables the MUL feature with most or all the functionality known on Wikidata.
• The feature is documented as stable / available somewhere.

Open questions:

• Which team is responsible for this? Wikidata / Omega? Suite? Cloud? Product Platform?