The space/time continuum can't take any more
jikuukanrentai koreijou taerarenai (時空間連体、これ以上、耐えられない)
Actually, the proper Japanese word for "continuum" is renzokutai(連続体), not rentai(連体) (trust me, I'm a physicist). But "jikuukan renzokutai" would be a tongue twister. Or alternatively, "jikuu renzokudai" would have been better one.
History needs Takezo Kensei to be a hero today...
rekisi ha kyou kensei takezou ga eiyu to site hituyou nanoni (歴史は今日、剣聖武蔵が英雄として必要なのに)
Hmm, this is awkward. It would be "歴史じゃ今日、剣聖武蔵が英雄になんなきゃいけないのに". BTW, the name Takezou must have been taken from Miyamoto Musashi, whose original name had been Shinmen Takezou.
Kensei and Bandits
The Boss of Bandits
is played by a Japanese actor/screenwriter, Mitsutake Kurando (His WEB page). His blog has some entries about Heroes shootings (in Japanese).
(Bandits' boss) And why does he think we will listen?
dare ga sono zaregoto wo kiku (誰がその戯言を聞く = Who'd listen to your gibberish)
(Hiro) Because Takezo Kensei is the greatest swordsman Japan has ever seen.
sore ha kensei takezou ga nihon ichi no samurai de aru kara (それは剣聖武蔵が日本一の侍であるから = literal translation)
The line of bandits' boss itself is an excellent Jidaigeki-sh translation (maybe Mitsutake did this). But here original question "Why" is changed to "Who", and Hiro's answer doesn't make sense.
I wouldn't do that...
Muda muda muda muda muda muda muda! (無駄無駄無駄無駄！)
Masi's a true geek! This is a signature cry of Dio Brando, the arch villain who can stop time, in Jojo's bizarre adventure Manga. According to this wikipedia entry, "muda muda muda" has become popular among English speaking geeks via this Flash movie, just like the "All Your Base" flash. Probably Masi'd thought that this phrase is popular enough for US viewers.
Before I take your clothes next
samonaito tugiha ifuku dazo (さもないと、次は衣服だぞ)
This is a bit awkward: we don't use "ifuku(衣服)" in daily life. It would be just "fuku" or "kimono". If I were him, I'd say "さもなくば次はフンドシまで頂くぞ" (or I'll take your underpants next).
The Battle of 12 Swords
12 furi no katana no tatakai (１２振りの刀の戦い)
When you enumerate objects in Japanese, you have to add proper suffix to the number, which depends on the kind of the object (http://en.wikipedia.org/wiki/Japanese_counter_word).
Not many Japanese knows the proper counter for katana is "furi".
Boku/Watashi/Ore
Usually Hiro referes to himself as "Boku (僕)" which is common for geek-types. When speaking to the bandits as Kensei, he uses more formal "Watashi (私)" (maybe true samurai uses "Sessha (拙者)"). Later on, when he talks to Yaeko, he also uses "Ore (俺)" which is suitable for badass. Apparently Hiro is self-absorbed at that moment :-).
ma ikkaa(ま、いっかー)
Short for "maa ii kana?" and means "Although it's not what I expected, I think I can accept it".
