I doubt it's coincidental. Gunbuster is secretly Japanese imperialist power fantasy, which is mostly expressed in-universe in subtle ways that an American viewer would never notice or care about.
For example (this is the first thing I found on Google), someone wrote a series of blog posts on this subject, and in their
sum-up of episode 6, they briefly discuss how Hawaii is portrayed during its short appearance in this episode. In short, there are a number of advertisements in Japanese, for Japanese goods. Why is that?
So there you go! They aren't using Japanese in Gunbuster (just) out of convenience; Japan is the leading superpower in the series.