REUSE is an initiative by FSFE, which intends to make licensing easier with best practices to display legal information through comment headers on source files that can be easily human and machine readable.

Because these files are new and translation shall involve a lot of people as copyright holders, now is the best time to adopt REUSE. As a first implementation, this commit intentionally limits the scope to i18n directory.

Cherry-picked from c737ed0612 and a2445af6d0

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3841

Most of the preparation for this was done by Suguru Hirahara (https://github.com/luixxiul).
I've merely reorganized/polished the scripts and instructions in the `i18n/` directory.

While translations can happen even now, more work is necessary to

- make the translation flow better (integrating Weblate), etc.

- restore the Github Actions workflows that Suguru Hirahara had already developed to
  adapt them to our new workflow