[Spike] Scoping - create fallback-to-English system for localized tech docs site

High level goal: Implement a system to fall back to English content when Japanese translations are not available. Document how fallback works

Lower level goal: Need to scope the issue further. What would the engineering effort be? What questions do we need to pose and answer before understanding the engineering scope? What are specific examples of the UX of fallback, when translation is not straightforward, such as in the case of auto generated pages content?

Related issue: https://gitlab.com/gitlab-com/localization/docs-site-localization/-/issues/27+

cc @jcole-ext @laurenbarker @rasamhossain @mjsibanez