[i18n] Link to external docs in proper locale

[chalin]

Originally posted by @chalin in #6197 (comment):

Oh! Good idea to link to the Japanese translation of the GitHub docs page. Let me see if I can fix this in the en version using a param.

The link being references in the PR is:

• https://docs.github.com/ja/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork, vs
• https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork

I'm wondering if we can handle Wikipedia links like this too.

/cc @Msksgm