Add a note on Ja pages that are not translated

[amarantha-k]

This issue captures an actionable task from a PR review process. (https://github.com/XRPLF/xrpl-dev-portal/pull/2816)

Currently, when an English page is not translated, the page appears in English even when accessed via the /ja/ link.

For such pages, it would be good to add a note at the top of the page with text such as "This page needs to be translated. Would you like to contribute? <link to translation guide?>"

[tequdev]

This can be achieved if realm can obtain the language and file path of the content currently being displayed.

Japanese pages that are not currently translated appear as follows in schema.ts.

config.variables.lang: "ja"
node.location.file: "@l10n/ja/docs/references/protocol/transactions/types/delegateset.md"

I want to get “en-US” for these or other properties.