Add missing i18n support in header link

[cheercroaker]

Hello and thank you for this project!

Motivation

I have noticed that the link to go to the "main" page in the navigation header (the one in the upper-left corner, with the header icon) was always targeting the page in the default—en—language, even if localization is enabled and another language was selected (e.f. fr).
I think this is something that has been forgotten as I expected the link to take the currently selected language into account.

Have you read the Contributing Guidelines on pull requests?

Yes.

As this is a very small modification, I quickly tried to set up a local working environment to be able to run "prettier" and "Jest tests", but could not get it working within a 2 mn time box so I admit I skipped those steps.
I made sure the line was still < 80 characters (which was not before I read the Contributing Guidelines) and had the right number of spaces for indentation though.
Disclaimer: I am not a JavaScript/JSX developer.

Test Plan

1. Have a site with translation enabled and at least another language in addition to English.

2. Go to the main page of the site

3. Change language with the drop-down on the upper-right corner

4. Check that the link to the main page in the upper-left corner,—in the navigation header, the one with the header icon—correctly targets the main page in the selected language

Related PRs

None (as far as I've looked for).

[cheercroaker]

ci/circleci test initially failed because of the prettier check, so I finally was able to run prettier locally to format the file according to the formatting rules.

(npm run prettier does not work on my configuration, it's saying No such file or directory: 'format:source',—whether I use npm or yarn directly—so I searched a bit more and found I could do npm run format:source directly after having installed prettier and to format the file with it, hence my second commit)

[JoelMarcey]

This looks good. Thanks!

[JoelMarcey]

@cheercroaker Thanks for this PR. Good catch 😄