Tetsu Nakamura, who plays Dr. Suzuki in THE MANSTER (and was sometimes billed as Satoshi Nakamura), spoke fluent English (he was born in Canada) and was in tons of Toho monster/sci-fi films we know and love, some of which featured western actors (e.g. MOTHRA), but also in a lot of co-productions where his English skills came in handy, e.g. LATITUDE ZERO, RED SUN, THE LAST DINOSAUR. He was in the Breakston-involved Japanese co-productions including the aforementioned GEISHA GIRL and something with Martha Hyer called ORIENTAL EVIL (1951).
I first took note of Nakamura in a movie called TOKYO FILE 212, an independent U.S.-Japan co-production with two top-billed American stars, Florence Marly (QUEEN OF BLOOD, TOKYO JOE) and Lee Frederick, and a Japanese supporting cast, that was distributed in the U.S. by RKO. It's a B-spy thriller, but was shot entirely on location in Japan. I bought it on VHS at Kim's Video several years ago.
Nakamura was also in a film I'm eager to see called FUTARI NO HITOMI (1952), which pairs former U.S. child star Margaret O'Brien (MEET ME IN ST. LOUIS) with the then-reigning teen recording star in Japan, Hibari Misora, who was in a 1955 color musical called JANKEN MUSUME, which I've seen and written about on my J-pop blog.
Here's a shot of Misora in JANKEN MUSUME (she's on the right):
I've seen THE MANSTER. It's not a good movie, but it's a fascinating one. I don't remember my criticisms of it, but I think it needed a more interesting lead actor for it to have worked. I don't remember if it had a truncated ending or not. I believe I taped it off a pre-TCM TNT.