feedback5 WI: docs/I18N.md (translating, adding strings, rebuilding the EN game texts, phase 2) and credits

NOTICE.md and THIRD-PARTY-NOTICES.md credit @YuriRestia (PR #70) for the English UI strings and fallback
translations, note that PR #70's ak-spa-database.pages.dev texts are not used and that arknights.wiki.gg is
CC BY-SA 4.0, and name the EN data mirrors (ArknightsAssets/ArknightsGamedata, Kengxxiao/ArknightsGameData_YoStar).
DATA.md: data/i18n/en.json is written by tools/build-i18n.mjs (rerun after a text-changing build).
PLAYING.md: the 中文 / English switch.

docs/DATA.md

@@ -1,8 +1,11 @@
 # DATA.md — generated game data (`data/*.json`)
 
-All files in `data/` except `data/assets.json` are produced by **`node tools/build-data.mjs`** (task F1) from the
+All files in `data/` except `data/assets.json` and `data/i18n/` are produced by **`node tools/build-data.mjs`** (task F1) from the
 official zh_CN client data ([Kengxxiao/ArknightsGameData](https://github.com/Kengxxiao/ArknightsGameData)) joined
 with `docs/research/*.json`. Do not edit them by hand — change the build script and rebuild.
+`data/i18n/en.json` (the official English texts of these files by record id and field) is written by
+`node tools/build-i18n.mjs` — rerun it after a build that changes a text (docs/I18N.md §2; `test/i18n-data.test.js`
+fails while it is stale).
 `data/assets.json` is written by `tools/fetch-assets.mjs`, which keeps the current file rather than drop entries whose
 downloads failed on this machine unless `--allow-shrink` (or `--prune`) is passed (docs/ASSETS.md, DESIGN §21.25).