Hi Guix!

As discussed with Ludo on IRC, new translations for the manual and
cookbook are a bit annoying, because you need to build them regularly.
Ludo proposed to implement a threshold to ensure we're not compiling
what is mostly the English manual. Here's the proposal:

manual and cookbook: only include new languages when they reach 10%
completion. Remove languages when they fall below 5%.

website (unrelated to this repo, but still important): only include new
languages when they reach 80% completion. Remove languages when they
fall below 60%. The reason for the higher threshold is that the website
acts as some sort of advertisement, so we want a higher quality than
half English half translated.

guix and packages: this is not an issue for developpers or users to
have more, so no threshold (other than at least one string needs to be
translated). Removal of obsolete translations (that do not have any
relevant strings anymore) is not implemented in the series.

The first patch documents the thresholds in the manual. The second
patch fixes the download-po target to make it enforce the threshold and
the last patch removes files that are under the 5% threshold for the
manual and cookbook.

In the long run, we might want to find a way to not build the
translated manuals by default...