i18n (internationalization): support non-english translation sets for the Backrest UI

[garethgeorge]

Is your feature request related to a problem? Please describe.

Backrest UI is english only today but it's increasingly apparent that the project is used in a range of locales -- it'd be great to provide native language translations for those users.

Describe the solution you'd like Use https://react.i18next.com/getting-started to provide support for translation sets in the WebUI. As a first step Backrest should provide a string-set for english but should indirect all UI string references through an internationalization library.

As a followup I'd like to aim to solicit PRs for translation sets for the community (and may seed a few via Google translate).

Additional context

I'm a native english speaker so if you're fluent in another language and interested in this feature OR interested in supporting translations engagement is definitely appreciated