Standardize on 'en', not 'en-US' as a language code, but register both

[eyeseast]

Trying to solve #818

My browser sets the accept-language header and window.navigator.language to en-US, but we weren't registering that as a locale, only en. So this adds that registration and also standardizes our own setting to en.

[github-actions[bot]]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	30.9%	6727 / 21766
🔵	Statements	30.9%	6727 / 21766
🔵	Functions	49.72%	274 / 551
🔵	Branches	75.65%	1013 / 1339

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/lib/components/documents/Access.svelte	100%	100%	100%	100%
src/lib/components/documents/BulkActions.svelte	0%	0%	0%	0%	1-113
src/lib/i18n/index.js	100%	66.66%	100%	100%
src/lib/utils/language.ts	100%	100%	100%	100%
src/lib/utils/tests/language.test.ts	100%	100%	100%	100%
src/routes/+layout.ts	0%	0%	0%	0%	1

Generated in workflow #770 for commit dd5672e by the Vitest Coverage Report Action

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