Support fallback to source locale

[yunsii]

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

In a big project, if someone do not extract and translate messages, it will be render as message id directly in production env. It is unreasonable I think.

Describe proposed solution

Is it possible to use source locale message to render after build? Or report un-translated messages at some time.

[timofei-iatsenko]

Add extracting to your build step before building app, something like build: "lingui extract && next build" this way catalogs would always be fulfilled with original messages.

[yunsii]

It is only works for source locale, fulfilled with original messages. But not work for others locales.