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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codes/java/chapter_divide_and_conquer/fast_power.java

@@ -0,0 +1,27 @@
+/**
+ * File: fast_power.java
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+package chapter_divide_and_conquer;
+
+public class fast_power {
+    /* 快速幂 */
+    static int fastPow(int x, int n) {
+        if (n == 0) {
+            return 1;
+        }
+        int half = fastPow(x, n / 2);
+        if (n % 2 == 0) {
+            return half * half;
+        }
+        return half * half * x;
+    }
+
+    public static void main(String[] args) {
+        assert fastPow(7, 0) == 1;
+        assert fastPow(3, 5) == 243;
+        assert fastPow(2, 6) == 64;
+    }
+}