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/csharp/chapter_divide_and_conquer/fast_power.cs

@@ -0,0 +1,28 @@
+/**
+ * File: fast_power.cs
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+namespace hello_algo.chapter_divide_and_conquer;
+
+public class fast_power {
+    /* Exponentiation by squaring */
+    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;
+    }
+
+    [Test]
+    public void Test() {
+        Assert.That(FastPow(7, 0), Is.EqualTo(1));
+        Assert.That(FastPow(3, 5), Is.EqualTo(243));
+        Assert.That(FastPow(2, 6), Is.EqualTo(64));
+    }
+}