[i18n] Ensure fallback pages have en lang

[chalin]

• Fixes [i18n] Ensure that fallback pages have en lang attribute #4664
• Adds partial functions used to determine if a page is a fallback page, and what the true lang code is for such pages
• Adds a Docsy override of layouts/docs/baseof.html
• Non-functional tweaks to scripts/check-i18n.sh

Before and after

$ curl -s https://opentelemetry.io/ja/docs/getting-started/ | perl -nle 'print $1 if /html[^>]*(lang=\S+)/i' 
lang=ja
$ curl -s https://deploy-preview-4683--opentelemetry.netlify.app/ja/docs/getting-started/ | perl -nle 'print $1 if /html[^>]*(lang=\S+)/i' 
lang=en

Preview

• E.g., https://deploy-preview-4683--opentelemetry.netlify.app/ja/docs/getting-started/