react-routerのバージョンを下げても日本語を期待どおりできない問題は再現しました
$ git diff package.json
diff --git a/package.json b/package.json
index 9ae3082605..a3a0c67632 100644
--- a/package.json
+++ b/package.json
@@ -112,8 +112,8 @@
"react-overlays": "^5.2.1",
"react-redux": "^8.0.4",
"react-redux-loading-bar": "^5.0.4",
- "react-router": "^5.3.4",
- "react-router-dom": "^5.3.4",
+ "react-router": "^5.2.1",
+ "react-router-dom": "^5.2.1",
"react-router-scroll-4": "^1.0.0-beta.1",
"react-select": "^5.7.3",
"react-sparklines": "^1.7.0",