Whenever the pirate guilds make any use of magic, they chant something repetitively (example: on episode 2, about 3 minutes in, as shown on Funimation. It's not subtitled, and kind of hard to make out.
I'd imagine it's something in Chinese; their powers are referred to as "Tao magic", with one character in particular called a "Tao master".
What is this chant, and its significance if any? Note that this is on the English dub, if it matters.
It is very much the case that the pirates are associated with the Chinese Guild. This much is said in the intro of episode 7.
This particular group of pirates are from the Hoppo system. They are called the 108 Suns, the most brutal member of the Chinese Guild.
The chant itself is not Chinese. It sounds more like faux Chinese, or at least something to sound as close to foreign as possible. The sub doesn't make any attempt to transliterate the chant at all, and any announced senjutsu attacks are nonsensical phrases in both Japanese and Chinese.