Update what-is-a-locale.md

[LaureBrosseau]

I have removed the locale properties, rearranged some text. Added a part "Do I need one locale enabled or more?"

[NolwennP]

What do we do with the French translation of this article that already exists? Should we create a ticket to remember to update the translation as well?

[LaureBrosseau]

Hi @NolwennP, I created an issue to handle this: #230 :)

[NolwennP]

I was ready to approve your PR, but I just saw that it's missing related articles. Can you add some? Like the Manage your locales one, the ones on the EE permissions... Thanks!