some minor modifications

src/index.js

@@ -1,18 +1,31 @@
-import React from "react";
+import React, { useState } from "react";
 import ReactDOM from "react-dom/client";
 import CircularProgress from "@mui/material/CircularProgress";
 import Divider from "@mui/material/Divider";
 import ReactMarkdown from "react-markdown";
 import Paper from "@mui/material/Paper";
+import Stack from "@mui/material/Stack";
+import Button from "@mui/material/Button";
 import { useFetch } from "./hooks/Fetch";
 import { I18N, URL_RAW_PREFIX } from "./config";
 
 function App() {
+  const [lang, setLang] = useState("zh");
   const [data, loading, error] = useFetch(
-    `${URL_RAW_PREFIX}/${I18N?.["about_md"]?.["zh"]}`
+    `${URL_RAW_PREFIX}/${I18N?.["about_md"]?.[lang]}`
   );
   return (
     <Paper sx={{ padding: 2, margin: 2 }}>
+      <Stack spacing={2} direction="row" justifyContent="flex-end">
+        <Button
+          variant="text"
+          onClick={() => {
+            setLang((pre) => (pre === "zh" ? "en" : "zh"));
+          }}
+        >
+          {lang === "zh" ? "ENGLISH" : "中文"}
+        </Button>
+      </Stack>
       <Divider>{`KISS Translator v${process.env.REACT_APP_VERSION}`}</Divider>
       {loading ? (
         <center>