What about the other American characters? There is Pegasus (speaks lots of English), Rebecca (says, "god damn!" way too much in Japanese version), the DOMA knights?, Johan?, Edo Phoenix? (says, "cemetary" and "effect" a lot (and they were from American academy in the manga. Idk
Some other characters, particularly Edo, don't have clear nationality despite their name, and their English could be attributed to how Japanese LOVE using English words despite being unable to fluently speak in English.
True. Some of them have very non Japanese-sounding names (Johan Anderson, Jim Cook). Edo is a bit of a wild card, though. The last name "Phoenix" doesn't sound Japanese, but his first name is because of the Edo period in Japan. His father worked for Pegasus and he is American, and he was American Academy in the manga. So maybe it is possible. But it is hard to tell sometimes. And DOMA, idk.