MNT Ensure default_locale to en_US

[emteknetnz]

Issue silverstripe/.github#357

For whatever reason, CMS 5 sink now uses cwp/cwp-core 3.x-dev instead of 3.0.2, and 3.x-dev sets the default_locale with yml rather than _config.php silverstripe/cwp-core@3.0.2...3

This causes a few i18n tests to fail https://github.com/silverstripe/recipe-kitchen-sink/actions/runs/12877846807/job/35902983644#step:12:334

Also causes a locale date test to fail - Failed asserting that '29/02/2020, 17:00' matches PCRE pattern "#2/29/20,\h5:00\hPM#iu". https://github.com/silverstripe/recipe-kitchen-sink/actions/runs/12877846807/job/35902983814

Note that CMS 6 doesn't have this issue as cwp-core is no longer in CMS 6 sink

New CI sink run with this PR and also this PR https://github.com/creative-commoners/recipe-kitchen-sink/actions/runs/12920534794

[GuySartorelli]

This will affect project unit tests as well - please make sure to add a quick note about this in the 5.4 changelog.

[GuySartorelli]

I've just realised why i18n::config()->get('default_locale') was being used - projects can of course run unit tests, and will likely want to run those tests against the default locale they have set for their project.

[GuySartorelli]

LGTM - will update the commit message on merge (since it's not only affecting our tests it should either be FIX or ENH)