There are several possibilities to the purpose of this comic. I choose to believe while Luigi Mario, being an orphan plumber (if you dont believe me watch the movie), would have a though time translating all the maundo (purposely made up word) words flying back and forth. However, he
1 has enough sense and intuition to guess at what Megaman is saying most of the time he speaks in a vocabulary beyond that of a foreigner orphan plumber. Luigi is comparably uneducated standing next to Megaman (pervious lab-bot and ten year old turned super fighter) and this new guy, but he is by no means stupid.
2 has been around Megaman enough to guess what he is saying CORECTLY.
Sure, Luigi catches on to most of the terms, but hes not gonna incorporate them into his speech patterns anytime soon.