[Feature] Translations!

Everything is in place, just every instance of text needs to be replaced with a `FormattedMessage` component (or otherwise) from `react-intl`.

Language could be set with e.g. `browser-locale` lib.

Totally fine to break this ticket into using FormattedMessage everywhere, and then creating new tickets for supporting specific languages.

Start by inventorying the React app's user-facing text and identify the entry point where locale selection can be configured with browser-locale. Review react-intl usage and replace the text instances with FormattedMessage or an equivalent approach. Done means the app's displayed text is internationalization-ready; support for specific languages is explicitly left for separate tickets.

Written by the indexing model from the issue text.