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/go/chapter_divide_and_conquer/fast_power.go

@@ -0,0 +1,17 @@
+// File: fast_power.go
+// Created Time: 2026-08-18
+// Author: Hello Algo Team
+
+package chapter_divide_and_conquer
+
+/* 高速べき乗 */
+func fastPow(x, n int) int {
+	if n == 0 {
+		return 1
+	}
+	half := fastPow(x, n/2)
+	if n%2 == 0 {
+		return half * half
+	}
+	return half * half * x
+}

ja/codes/go/chapter_divide_and_conquer/fast_power_test.go

@@ -0,0 +1,13 @@
+// File: fast_power_test.go
+// Created Time: 2026-08-18
+// Author: Hello Algo Team
+
+package chapter_divide_and_conquer
+
+import "testing"
+
+func TestFastPower(t *testing.T) {
+	if fastPow(7, 0) != 1 || fastPow(3, 5) != 243 || fastPow(2, 6) != 64 {
+		t.Fatal("fast power returned an unexpected value")
+	}
+}