https://github.com/jenkinsci/blueocean-plugin/blob/4a1ea8038ef3eec3c8cb5725050c888f97834292/blueocean-core-js/src/js/i18n/i18n.js#L16

{urlConfig.getJenkinsRootURL()}} is never null and always returns a trailing slash

{/}

in the case of dogfood urlConfig.getJenkinsRootURL() will always be {/}

. This causes the browser to call {{//i18n/resourceBundle?language=

{lng}&baseName={ns}} which says the double slash should match the protocol (http or https)

I think the fix is:
const prefix = urlConfig.getJenkinsRootURL()
const loadPath = `${prefix}i18n/resourceBundle?language={lng}

&baseName=

{ns}

`;