Error: Requested locale "en-US" was not prefetched

[houdmont]

ABT Version: 5.2.2

PHP Version: 7.4.14

Theme: Custom theme

Plugin List:

• Advanced Ads
• Autoptimize
• Braintree for WooCommerce
• WooCommerce
• Jetpack
• LearnDash
• Logic Hop
• MathML
• Ninja Forms
• Oasis Workflow
• WooCommerce
• Yoast
• WP Simple SAML

Browser: Google Chrome (Version: 88.0.4324.146)

Expected behavior: When citing an uncited item, I expected the item to appear in the editor as a citation (as well as in the reference list generated in the editor)

Actual behavior: The uncited item does appear in the list of cited items in the sidebar, but does not appear as a citation in the editor, nor does a reference list get generated.

Console messages:

processor.ts:60 Uncaught (in promise) Error: Requested locale "en-US" was not prefetched
    at Object.retrieveLocale (processor.ts:60)
    at p.Engine.localeConfigure (citeproc.js?ver=1610460369:1)
    at new p.Engine (citeproc.js?ver=1610460369:1)
    at new e (processor.ts:81)
    at actions.ts:106
    at l (index.js?ver=1.8.3:2)
    at Generator._invoke (index.js?ver=1.8.3:2)
    at Generator.next (index.js?ver=1.8.3:2)
    at redux-routine.min.js?ver=8ec41c688c5daad1d264d85374a1c8b0:2

processor.ts:89 Uncaught (in promise) TypeError: Cannot read property 'rebuildProcessorState' of undefined
    at e.value (processor.ts:89)
    at actions.ts:107
    at l (index.js?ver=1.8.3:2)
    at Generator._invoke (index.js?ver=1.8.3:2)
    at Generator.next (index.js?ver=1.8.3:2)
    at redux-routine.min.js?ver=8ec41c688c5daad1d264d85374a1c8b0:2

This behaviour occurs even with the plugins listed above disabled. This takes place on both new posts as well as historical posts that were authored before the plugin was in place.

[jenkoian]

As far as I can tell this is due to en_* always assuming en_US due to https://github.com/Juris-M/citeproc-js/blob/27c95b261ed695e02520fbb5e352151aa2794e88/src/load.js#L1103-L1146