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/PLAYING.md

@@ -4,6 +4,8 @@
 
 本文数值来自官方数据表（`data/config.json` 等）与 [research](research/00-INDEX.md)；游戏内左下角 📖「玩法说明」有官方教程图（随素材一起下载；没下载到时显示文字要点）。操作方式见 [README · 操作](../README.md#操作)。
 
+界面语言：默认中文；标题画面右上角或「设置」里可切换 **中文 / English**（会记住选择，不用刷新）。英文的干员、技能、盟约、装备、敌人等游戏文本取自官方英文服数据；尚未翻译的界面文字暂时显示中文（docs/I18N.md）。
+
 ## 1. 一局的流程
 
 ```