Translate the README into more languages

[Maxerns]

The root README is published in English, Spanish, French, German and
Brazilian Portuguese (see #160). Readers outside those languages only get
the English original.

What is wanted

Translations of README.md into additional languages, contributed by people
fluent in the language. There is no fixed list: if you are fluent in a
language we do not cover yet, it is wanted. Widely spoken developer
languages such as Italian, Dutch, Polish, Ukrainian, Japanese, Korean and
Simplified Chinese are obvious gaps, but any language is welcome.

To avoid duplicate work, comment on this issue to claim a language before
starting, and keep to one language per pull request. Reviews of the four
existing translations by fluent speakers are welcome too.

How to contribute

Copy an existing translation (for example README.es.md) to
README.<code>.md and follow the process in CONTRIBUTING.md
("Translating the README"):

• Translate prose only. Fenced code blocks, commands, identifiers, URLs,
  link destinations, inline code, heading levels, tables and admonition
  markers must stay byte-identical to the English file.
• Keep the language nav bar at the top, link the new file from every
  README, and register it in TRANSLATIONS in
  scripts/check-readme-i18n.
• Run scripts/check-readme-i18n before opening the pull request; it and
  the readme-i18n CI job must pass.

English (README.md) stays authoritative and wins on any discrepancy.