Add new localisations

[valeriocomo]

Context

Derived from requirement RF-PCE-002

Scope

Widen the editor's UI localisation so that every country contributing to the European
software catalogue can use it in its own language. Five locales are missing — Spanish,
Portuguese, Swedish, Finnish and Greek — and each is tracked as a sub-issue of this one,
so they can be picked up independently by different translators.

This issue covers the work that is not per-language:

• Make locale registration less manual. Each locale is currently wired in by hand,
  with both a JSON import and an entry in the resources map in src/i18n/index.ts.
  Doubling the number of locales is the moment to fix that, so adding a further language
  is a matter of dropping in a file.
• Catch incomplete translations automatically. Locales drift out of sync with
  en.json and nothing detects it — missing translations in French 🇫🇷 #429 and Check the Italian localization #270 both exist for that reason. A missing
  key should fail CI, not reach production as an untranslated string.
• State how translations are maintained. Ten locales cannot be kept current by
  ad-hoc effort; the project needs a documented path for contributing and reviewing a
  translation.
• Bring the existing locales up to the same bar (de, fr, it, nl), or file
  their gaps separately.

The requirement is about UI string coverage, not about the language metadata the form
offers for publiccode.yml itself, which already comes from locale-codes.

Affected areas

• src/i18n/index.ts — imports and the resources map; supportedLngs is derived from
  it, so the language picker and getSupportedLanguages() follow automatically.
• src/i18n/locales/ — currently de, en, fr, it, nl; en.json (~23 KB) is the
  reference for the key structure.
• .github/workflows/test.yml — where the completeness check should run.
• formatLanguageLabel() uppercases the Intl.DisplayNames output, which interacts
  with Use the target language in the language picker #538 and deserves a look once a non-Latin script is in the list.

Non-functional constraints

• RNF-PCE-006 — Long-term sustainability: this is the whole point of the
  infrastructure work above. Without a completeness check and a contribution policy,
  ten locales become ten sources of drift.

Acceptance criteria

• Adding a locale no longer requires editing the resources map by hand
• An automated check fails when a locale is missing keys present in en.json, and
  runs in CI
• The way to contribute and review a translation is documented
• Existing locales (de, fr, it, nl) pass the completeness check, or their
  gaps are filed separately
• All five sub-issues closed

Related

• missing translations in French 🇫🇷 #429 — missing French translations; the completeness check here would have caught it
• Check the Italian localization #270 — Italian localisation quality review
• Use the target language in the language picker #538 — language picker labels, same surface touched here