feat: close #2294 add documents for adding a new translation

Author: Yidadaa

docs/translation.md

@@ -0,0 +1,12 @@
+# How to add a new translation?
+
+Assume that we are adding a new translation for `new`.
+
+1. copy `app/locales/en.ts` to `app/locales/new.ts`;
+2. edit `new.ts`, change `const en: LocaleType = ` to `const new: PartialLocaleType`, and `export default new;`;
+3. edit `app/locales/index.ts`:
+4. `import new from './new.ts'`;
+5. add `new` to `ALL_LANGS`;
+6. add `new: "new lang"` to `ALL_LANG_OPTIONS`;
+7. translate the strings in `new.ts`;
+8. submit a pull request, and the author will merge it.