Add a second set of translation files to override Django defaults

[philgyford]

There are some words that we want to keep in English on other languages' sites. e.g. "Home" and "Search" in the navbar.

Even if setting the translations for these in the languages' .po files like:

msgid "Search"
msgstr "Search"

the terms often get translated into the local language. Because Django's own "helpful" translations into local languages seem to override our own.

But, for some reason, adding a second set of translation files (app/local_extra/) and adding these non-translations into those, forces the words into English.

From https://stackoverflow.com/a/41945558/250962

For #360

[StephenAbbott]

@philgyford This PR looks to be fixing the top navbar issue by making the words show up in English as expected (see French page from staging https://openownership.hactar.work/fr/publications/aspects-a-prendre-en-compte-relativement-a-elaboration-de-bases-de-donnees-relationnelles-pour-les-informations-sur-la-propriete-effective/)

But the 'View in' and 'Language' buttons are still showing French when we want them to show Français - see https://github.com/openownership/openownership.org-wagtail/pull/310/files#diff-1a8fe0e6b0631821238d60f7aa085a73a6186b10139e86e20489280d8a42613cR30

Wherever English is written, we want it to say English, French should always show up as Français on every language version, Spanish as Español everywhere, German as Deutsch everywhere, etc...

Am I using the wrong file or filling in the .po incorrectly in terms of wanting to update the names of the languages used for the language buttons?

[philgyford]

@StephenAbbott The idea is that to override these "stubborn" settings, you add the non-translations (English phrases that should appear in English in every translation) into the files in locale_extra/. Does that help?