Add internationalization

[hackartisan]

Acceptance criteria

• [x] gettext is in the deps
• [ ] there's documentation in our readme about how to use it when you're adding a string in a template
• [ ] there's documentation on what to do if you edit a string
• [ ] The home page text is translated to one language - spanish.
• [ ] The language dropdown works.

First Step

Read the gettext getting started documentation.

Technical Implementation Tips

We'll probably have to wrap the text in the home page that's there now in some kind of gettext tag.

[sdellis]

Inspiration for Plug code: https://github.com/goofansu/locale_plug/blob/main/lib/locale_plug.ex

Look at Restore Locale code so we can by DRY: https://hexdocs.pm/phoenix_live_view/using-gettext.html

[sdellis]

Current translations: https://github.com/pulibrary/lae-blacklight/tree/main/config/locales