Add multilingual exercise code (#1959)

Replace the exercise pages' Python-only snippets with source-backed implementations for the 13 visible programming languages, and localize reader-facing comments by site language. Zig remains out of scope.

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en/codes/javascript/chapter_computational_complexity/complexity_exercises.js

@@ -0,0 +1,63 @@
+/**
+ * File: complexity_exercises.js
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+/* Iterative summation */
+function sumIter(n) {
+    let res = 0;
+    for (let i = 1; i <= n; i++) {
+        res += i;
+    }
+    return res;
+}
+
+/* Recursive summation */
+function sumRecur(n) {
+    if (n === 1) {
+        return 1;
+    }
+    return n + sumRecur(n - 1);
+}
+
+/* Linear loop */
+function linearLoop(n) {
+    let res = 0;
+    for (let i = 0; i < n; i++) {
+        res += i;
+    }
+    return res;
+}
+
+/* Quadratic loop */
+function quadraticLoop(n) {
+    let res = 0;
+    for (let i = 0; i < n; i++) {
+        for (let j = i; j < n; j++) {
+            res += j;
+        }
+    }
+    return res;
+}
+
+/* Logarithmic loop */
+function logarithmicLoop(n) {
+    while (n > 1) {
+        n = Math.floor(n / 2);
+    }
+    return n;
+}
+
+if (
+    sumIter(1) !== 1 ||
+    sumRecur(1) !== 1 ||
+    sumIter(4) !== 10 ||
+    sumRecur(4) !== 10 ||
+    linearLoop(4) !== 6 ||
+    quadraticLoop(4) !== 20 ||
+    logarithmicLoop(4) !== 1 ||
+    logarithmicLoop(5) !== 1
+) {
+    throw new Error('complexity exercise check failed');
+}