バンサンです。

M17NされたIRBは、LANG=ja_JP.utf8でない場合、エラーが発生されます:
~% irb-1.9
/usr/local/lib/ruby-1.9/1.9.1/irb/locale.rb:126:in `load': No such
file to load -- irb/encoding_aliases.rb (LoadError)
	from /usr/local/lib/ruby-1.9/1.9.1/irb/locale.rb:37:in `initialize'
	from /usr/local/lib/ruby-1.9/1.9.1/irb/init.rb:114:in `new'
	from /usr/local/lib/ruby-1.9/1.9.1/irb/init.rb:114:in `init_config'
	from /usr/local/lib/ruby-1.9/1.9.1/irb/init.rb:16:in `setup'
	from /usr/local/lib/ruby-1.9/1.9.1/irb.rb:53:in `start'
	from /usr/local/bin/irb-1.9:12:in `<main>'

~% export LANG=ja_JP.utf-8
~% irb-1.9
irb(main):001:0>