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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ja/codes/javascript/chapter_divide_and_conquer/fast_power.js

@@ -0,0 +1,21 @@
+/**
+ * File: fast_power.js
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+/* 高速べき乗 */
+function fastPow(x, n) {
+    if (n === 0) {
+        return 1;
+    }
+    const half = fastPow(x, Math.floor(n / 2));
+    if (n % 2 === 0) {
+        return half * half;
+    }
+    return half * half * x;
+}
+
+if (fastPow(7, 0) !== 1 || fastPow(3, 5) !== 243 || fastPow(2, 6) !== 64) {
+    throw new Error('fast power check failed');
+}