Looks like the English writing says "Yoshitani" (maybe "Col. T. Yoshitani", or "Col. J. Yoshitani"). It would indeed be strange to sign a very patriotic flag using the alphabet of the enemy. Since there are no apparent German signatures on the flag, it is quite unusual. I can't quite get the large letters on the right; something 必勝 (something "certain victory")
In all other respects it looks genuine. Most of the flags we see here usually look funny, modern, ultra fake, or otherwise suspicious. This flag looks interesting - a ton of signatures, all written with various brushes and styles, no fake names (like Tojo Hideki), no anachronisms or obvious Chinese spellings, names of presumably real people with real ranks that (again, presumably) can be researched and verified, etc. It really looks fine except for the English signature. As Nagashima-san suggests, maybe the English writing was added later? Or maybe it was signed before the anti-Western sentiment really started ramping up? (Pre-Pearl Harbor)?