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/csharp/chapter_computational_complexity/complexity_exercises.cs

@@ -0,0 +1,66 @@
+/**
+ * File: complexity_exercises.cs
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+namespace hello_algo.chapter_computational_complexity;
+
+public class complexity_exercises {
+    /* 反復による総和 */
+    int SumIter(int n) {
+        int res = 0;
+        for (int i = 1; i <= n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* 再帰による総和 */
+    int SumRecur(int n) {
+        if (n == 1) {
+            return 1;
+        }
+        return n + SumRecur(n - 1);
+    }
+
+    /* 線形時間のループ */
+    int LinearLoop(int n) {
+        int res = 0;
+        for (int i = 0; i < n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* 二次時間のループ */
+    int QuadraticLoop(int n) {
+        int res = 0;
+        for (int i = 0; i < n; i++) {
+            for (int j = i; j < n; j++) {
+                res += j;
+            }
+        }
+        return res;
+    }
+
+    /* 対数時間のループ */
+    int LogarithmicLoop(int n) {
+        while (n > 1) {
+            n /= 2;
+        }
+        return n;
+    }
+
+    [Test]
+    public void Test() {
+        Assert.That(SumIter(1), Is.EqualTo(1));
+        Assert.That(SumRecur(1), Is.EqualTo(1));
+        Assert.That(SumIter(4), Is.EqualTo(10));
+        Assert.That(SumRecur(4), Is.EqualTo(10));
+        Assert.That(LinearLoop(4), Is.EqualTo(6));
+        Assert.That(QuadraticLoop(4), Is.EqualTo(20));
+        Assert.That(LogarithmicLoop(4), Is.EqualTo(1));
+        Assert.That(LogarithmicLoop(5), Is.EqualTo(1));
+    }
+}

ja/codes/csharp/chapter_divide_and_conquer/fast_power.cs

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+/**
+ * File: fast_power.cs
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+namespace hello_algo.chapter_divide_and_conquer;
+
+public class fast_power {
+    /* 高速べき乗 */
+    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));
+    }
+}