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/ruby/chapter_divide_and_conquer/fast_power.rb

@@ -0,0 +1,21 @@
+=begin
+File: fast_power.rb
+Created Time: 2026-08-18
+Author: Hello Algo Team
+=end
+
+### 高速べき乗 ###
+def fast_pow(x, n)
+  return 1 if n == 0
+
+  half = fast_pow(x, n / 2)
+  return half * half if n.even?
+
+  half * half * x
+end
+
+if __FILE__ == $0
+  raise 'fast power check failed' unless fast_pow(7, 0) == 1
+  raise 'fast power check failed' unless fast_pow(3, 5) == 243
+  raise 'fast power check failed' unless fast_pow(2, 6) == 64
+end